Molecular details of phage phi6 RNA-dependent RNA synthesis

For most RNA viruses RNA-dependent RNA polymerases (RdRPs) encoded by the virus are responsible for the entire RNA metabolism. Thus, RdRPs are critical components in the viral life cycle. However, it is not fully understood how these important enzymes function during viral replication. Double-stranded RNA (dsRNA) viruses perform the synthesis of their RNA genome within a proteinacous viral particle containing an RdRP as a minor constituent. The phi6 bacteriophage is the best-studied dsRNA virus, providing an excellent background for studies of its RNA synthesis. The purified recombinant phi6 RdRP is highly active in vitro and it possesses both RNA replication and transcription activities. The crystal structure of the phi6 polymerase, solved in complex with a number of ligands, provides a working model for detailed in vitro studies of RNA-dependent RNA polymerization. In this thesis, the primer-independent initiation of the phi6 RdRP was studied in vitro using biochemical and structural methods. A C-terminal, four-amino-acid-long loop protruding into the central cavity of the phi6 RdRP has been suggested to stabilize the incoming nucleotides of the initiation complex formation through stacking interactions. A similar structural element has been found from several other viral RdRPs. In this thesis, this so-called initiation platform loop was subjected to site-directed mutagenesis to address its role in the initiation. It was found that the initiation mode of the mutants is primer-dependent, requiring either an oligonucleotide primer or a back-priming initiation mechanism for the RNA synthesis. The crystal structure of a mutant RdRP with altered initiation platform revealed a set of contacts important for primer-independent initiation. Since phi6 RdRP is structurally and functionally homologous to several viral RdRPs, among them the hepatitis C virus RdRP, these results provide further general insight to understand primer-independent initiation. In this study it is demonstrated that manganese phasing could be used as a practical tool for solving structures of large proteins with a bound manganese ion. The phi6 RdRP was used as a case study to obtain phases for crystallographic analysis. Manganese ions are naturally bound to the phi6 RdRP at the palm domain of the enzyme. In a crystallographic experiment, X-ray diffraction data from a phi6 RdRP crystal were collected at a wavelength of 1.89 Å, which is the K edge of manganese. With this data an automatically built model of the core region of the protein could be obtained. Finally, in this work terminal nucleotidyl transferase (TNTase) activity of the phi6 RdRP was documented in the isolated polymerase as well as in the viral particle. This is the first time that such an activity has been reported in a polymerase of a dsRNA virus. The phi6 RdRP used uridine triphosphates as the sole substrate in a TNTase reaction but could accept several heterologous templates. The RdRP was able to add one or a few non-templated nucleotides to the 3' end of the single- or double-stranded RNA substrate. Based on the results on particle-mediated TNTase activity and previous structural information of the polymerase, a model for termination of the RNA-dependent RNA synthesis is suggested in this thesis.

Sequence Specific Interaction between Promoter DNA and Escherichia coli RNA Polymerase: Comparative Thermodynamic Analysis with One Immobilized Partner

Sequence specific interaction between DNA and protein molecules has been a subject of active investigation for decades now. Here, we have chosen single promoter containing bacteriophage Delta D-III T7 DNA and Escherichia coli RNA polymerase and followed their recognition at the air-water interface by using the surface plasmon resonance (SPR) technique, where the movement of one of the reacting species is restricted by way of arraying them on an immobilized support. For the Langmuir monolayer studies, we used a RNA polymerase with a histidine tag attached to one of its subunits, thus making it an xcellent substrate for Ni(II) ions, while the SPR Studies were done using biotin-labeled DNA immobilized on a streptavidin-coated chip. Detailed analysis of the thermodynamic parameters as a function of concentration and temperature revealed that the interaction of RNA polymerase with T7 DNA is largely entropy driven (83 (+/- 12) kcal mol(-1)) with a positive enthalpy of 13.6 (+/- 3.6) kcal mol(-1), The free energy of reaction determined by SPR and Langmuir-Blodgett technique was -11 (+/- 2) and -15.6 kcal mol(-1), respectively. The ability of these methods to retain the specificity of the recognition process was also established.

Synthesis of leader RNA and editing of P mRNA during transcription by rinderpest virus

Purified rinderpest virus was earlier shown to transcribe in vitro, all virus-specific mRNAs with the promoter-proximal N mRNA being the most abundant. Presently, this transcription system has been shown to synthesize full length monocistronic mRNAs comparable to those made in infected cells. Small quantities of bi- and tricistronic mRNAs are also synthesized. Rinderpest virus synthesizes in vitro, a leader RNA of not, vert, similar 55 nucleotides in length. Purified rinderpest virus also exhibits RNA editing activity during the synthesis of P mRNA as shown by primer extension analysis of the mRNA products.

Mechanism of RNA Translocation by a Viral Packaging Motor

Molecular motors are proteins that convert chemical energy into mechanical work. The viral packaging ATPase P4 is a hexameric molecular motor that translocates RNA into preformed viral capsids. P4 belongs to the ubiquitous class of hexameric helicases. Although its structure is known, the mechanism of RNA translocation remains elusive. Here we present a detailed kinetic study of nucleotide binding, hydrolysis, and product release by P4. We propose a stochastic-sequential cooperative model to describe the coordination of ATP hydrolysis within the hexamer. In this model the apparent cooperativity is a result of hydrolysis stimulation by ATP and RNA binding to neighboring subunits rather than cooperative nucleotide binding. Simultaneous interaction of neighboring subunits with RNA makes the otherwise random hydrolysis sequential and processive. Further, we use hydrogen/deuterium exchange detected by high resolution mass spectrometry to visualize P4 conformational dynamics during the catalytic cycle. Concerted changes of exchange kinetics reveal a cooperative unit that dynamically links ATP binding sites and the central RNA binding channel. The cooperative unit is compatible with the structure-based model in which translocation is effected by conformational changes of a limited protein region. Deuterium labeling also discloses the transition state associated with RNA loading which proceeds via opening of the hexameric ring. Hydrogen/deuterium exchange is further used to delineate the interactions of the P4 hexamer with the viral procapsid. P4 associates with the procapsid via its C-terminal face. The interactions stabilize subunit interfaces within the hexamer. The conformation of the virus-bound hexamer is more stable than the hexamer in solution, which is prone to spontaneous ring openings. We propose that the stabilization within the viral capsid increases the packaging processivity and confers selectivity during RNA loading. Finally, we use single molecule techniques to characterize P4 translocation along RNA. While the P4 hexamer encloses RNA topologically within the central channel, it diffuses randomly along the RNA. In the presence of ATP, unidirectional net movement is discernible in addition to the stochastic motion. The diffusion is hindered by activation energy barriers that depend on the nucleotide binding state. The results suggest that P4 employs an electrostatic clutch instead of cycling through stable, discrete, RNA binding states during translocation. Conformational changes coupled to ATP hydrolysis modify the electrostatic potential inside the central channel, which in turn biases RNA motion in one direction. Implications of the P4 model for other hexameric molecular motors are discussed.

Recovery of Yeast Saccharomyces cerevisiae after Thermal Insult

All organisms have evolved mechanisms to acquire thermotolerance. A moderately high temperature activates heat shock genes and triggers thermotolerance towards otherwise lethal high temperature. The focus of this work is the recovery mechanisms ensuring survival of Saccharomyces cerevisiae yeast cells after thermal insult. Yeast cells, first preconditioned at 37˚C, can survive a short thermal insult at 48-50˚C and are able to refold heat-denatured proteins when allowed to recover at physiological temperature 24˚C. The cytoplasmic chaperone Hsp104 is required for the acquisition of thermotolerance and dissolving protein aggregates in the cytosol with the assistance of disaccharide trehalose. In the present study, Hsp104 and trehalose were shown to be required for conformational repair of heat-denatured secretory proteins in the endoplasmic reticulum. A reporter protein was first accumulated in the lumen of endoplasmic reticulum and heat-denatured by thermal insult, and then failed to be repaired to enzymatically active and secretion-competent conformation in the absence of Hsp104 or trehalose. The efficient transport of a glycoprotein CPY, accumulated in the endoplasmic reticulum, to the vacuole after thermal insult also needed the presence of Hsp104 and trehalose. However, proteins synthesized after thermal insult at physiological temperature were secreted with similar kinetics both in the absence and in the presence of Hsp104 or trehalose, demonstrating that the secretion machinery itself was functional. As both Hsp104 and trehalose are cytosolic, a cross-talk between cytosolic and luminal chaperone machineries across the endoplasmic reticulum membrane appears to take place. Global expression profiles, obtained with the DNA microarray technique, revealed that the gene expression was shut down during thermal insult and the majority of transcripts were destroyed. However, the transcripts of small cytosolic chaperones Hsp12 and Hsp26 survived. The first genes induced during recovery were related to refolding of denatured proteins and resumption of de novo protein synthesis. Transcription factors Spt3p and Med3p appeared to be essential for acquisition of full thermotolerance. The transcription factor Hac1p was found to be subject to delayed up-regulation at mRNA level and this up-regulation was diminished or delayed in the absence of Spt3p or Med3p. Consequently, production of the chaperone BiP/Kar2p, a target gene of Hac1p, was diminished and delayed in Δspt3 and Δmed3 deletion strains. The refolding of heat-denatured secretory protein CPY to a transport-competent conformation was retarded, and a heat-denatured reporter enzyme failed to be effectively reactivated in the cytoplasm of the deletion strains.

Molecular mechanisms of bacteriophage phi6 RNA-dependent RNA polymerase and its utilization in biotechnology

Double-stranded RNA (dsRNA) viruses encode only a single protein species that contains RNA-dependent RNA polymerase (RdRP) motifs. This protein is a central component in the life cycle of a dsRNA virus, carrying out both RNA transcription and replication. The architecture of viral RdRPs resembles that of a 'cupped right hand' with fingers, palm and thumb domains. Those applying de novo initiation have additional structural features, including a flexible C-terminal domain that constitutes the priming platform. Moreover, viral RdRPs must be able to interact with the incoming 3'-terminus of the template and position it so that a productive binary complex is formed. Bacteriophage phi6 of the Cystoviridae family is to date one of the best studied dsRNA viruses. The purified recombinant phi6 RdRP is highly active in vitro and possesses both RNA replication and transcription activities. The extensive biochemical observations and the atomic level crystal structure of the phi6 RdRP provides an excellent platform for in-depth studies of RNA replication in vitro. In this thesis, targeted structure-based mutagenesis, enzymatic assays and molecular mapping of phi6 RdRP and its RNA were used to elucidate the formation of productive RNA-polymerase binary complexes. The positively charged rim of the template tunnel was shown to have a significant role in the engagement of highly structured ssRNA molecules, whereas specific interactions further down in the template tunnel promote ssRNA entry to the catalytic site. This work demonstrated that by aiding the formation of a stable binary complex with optimized RNA templates, the overall polymerization activity of the phi6 RdRP can be greatly enhanced. Furthermore, proteolyzed phi6 RdRPs that possess a nick in the polypeptide chain at the hinge region, which is part of the extended loop, were better suited for catalysis at higher temperatures whilst favouring back-primed initiation. The clipped C-terminus remains associated with the main body of the polymerase and the hinge region, although structurally disordered, is involved in the control of C-terminal domain displacement. The accumulated knowhow on bacteriophage phi6 was utilized in the development of two technologies for the production of dsRNA: (i) an in vitro system that combines the T7 RNA polymerase and the phi6 RdRP to generate dsRNA molecules of practically unlimited length, and (ii) an in vivo RNA replication system based on restricted infection with phi6 polymerase complexes in bacterial cells to produce virtually unlimited amounts of dsRNA. The pools of small interfering RNAs derived from dsRNA produced by these systems were validated and shown to efficiently decrease the expression of both exogenous and endogenous targets.

Purification of RNA polymerase from mycobacteria for optimized promoter-polymerase interactions

In vitro transcription analysis is important to understand the mechanism of transcription. Various assays for the analysis of initiation, elongation and termination form the basis for better understanding of the process. Purified RNA polymerase (RNAP) with high specific activity is necessary to carry out variety of these specific reactions. The RNAP purified from Mycobacterium smegmatis from exponential phase showed low promoter specificity in promoter-polymerase interaction studies. This is due to the presence of a large number of sigma factors during exponential phase and under-representation of sigma(A) required for house-keeping transcription. We describe an in vivo reconstitution of RNAP holoenzyme with sigma(A) and its purification, which resulted in holoenzyme with stoichiometric sigma(A) content. The reconstituted holoenzyme showed enhanced promoter-specific binding and promoter-specific-transcription activity compared to the enzyme isolated using standard procedure. Such in vivo reconstitution of stoichiometric holoenzyme could facilitate promoter-specific transcription assays, especially in organisms which encode a large number of sigma factors.

Cucumber chloroplast trnL(CAA) gene: nucleotide sequence and in vivo expression analysis in etiolated cucumber seedlings treated with benzyladenine and light.

The nucleotide sequence of a 714 bp BamHI-EcoRI fragment of cucumber chloroplast DNA was determined. The fragment contained a gene for tRNA(Leu) together with its flanking regions. The trnL(CAA) gene sequence is about 99% in similarity to broad bean, cauliflower, maize, spinach and tobacco corresponding genes. The relative expression level of the gene was determined by Northern (tRNA) gel blot and Northern (total cellular RNA) slot-blot analyses using the trnL gene probe in 6-day old etiolated cucumber seedlings and the seedlings that had been kept in the dark (dark-grown), treated with benzyladenine (BA) and kept in the dark (BA-treated dark-grown), illuminated (light-grown), and treated with BA and illuminated (BA-treated light-grown), for additional 4, 8 or 12 hr. The trnL transcripts and tRNA(Leu) levels in BA-treated dark-grown seedlings were 5 and 3 times higher, respectively after 4 hr BA treatment, while in the BA treated light-grown seedlings the level of trnL transcripts was only 3 times higher and had no detectable effect on mature tRNA(Leu) when compared to the time-4 hr dark-grown seedlings. However, the level of mature tRNA(Leu) did not show marked changes in the light-grown seedlings, whereas the level of trnL transcripts increases 3 times after 8 hr illumination of dark-grown seedlings. These data indicate that both light and cytokinin can signal changes in plastid tRNA gene expression. The possible regulatory mechanisms for such changes are discussed.

Cucumber chloroplast trnL(CAA) gene: nucleotide sequence and in vivo expression analysis in etiolated cucumber seedlings treated with benzyladenine and light

The nucleotide sequence of a 714 bp BamHI-EcoRI fragment of cucumber chloroplast DNA was determined. The fragment contained a gene for tRNA(Leu) together with its flanking regions. The trnL(CAA) gene sequence is about 99% in similarity to broad bean, cauliflower, maize, spinach and tobacco corresponding genes. The relative expression level of the gene was determined by Northern (tRNA) gel blot and Northern (total cellular RNA) slot-blot analyses using the trnL gene probe in 6-day old etiolated cucumber seedlings and the seedlings that had been kept in the dark (dark-grown), treated with benzyladenine (BA) and kept in the dark (BA-treated dark-grown), illuminated (light-grown), and treated with BA and illuminated (BA- treated light-grown), for additional 4, 8 or 12 hr. The trnL transcripts and tRNA(Leu) levels in BA-treated dark-grown seedlings were 5 and 3 times higher, respectively after 4 hr BA treatment, while in the BA treated light-grown seedlings the level of trnL transcripts was only 3 times higher and had not detectable effect on mature tRNA(Leu) when compared to the time-4 hr dark-grown seedlings. However, the level of mature tRNA(Leu) did not show marked changes in the light-grown seedlings, whereas the level of trnL transcripts increases 3 times after 8 hr illumination of dark-grown seedlings. These date indicate that both light and cytokinin can signal changes in plastid tRNA gene expression. The possible regulatory mechanisms for such changes are discussed.

mRNA and microRNA analysis reveals modulation of biochemical pathways related to addiction in the ventral tegmental area of methamphetamine self-administering rats

Background Methamphetamine is a highly addictive central nervous system stimulant with increasing levels of abuse worldwide. Alterations to mRNA and miRNA expression within the mesolimbic system can affect addiction-like behaviors and thus play a role in the development of drug addiction. While many studies have investigated the effects of high-dose methamphetamine, and identified neurotoxic effects, few have looked at the role that persistent changes in gene regulation play following methamphetamine self-administration. Therefore, the aim of this study was to identify RNA changes in the ventral tegmental area following methamphetamine self-administration. We performed microarray analyses on RNA extracted from the ventral tegmental area of Sprague–Dawley rats following methamphetamine self-administration training (2 h/day) and 14 days of abstinence. Results We identified 78 miRNA and 150 mRNA transcripts that were differentially expressed (fdr adjusted p < 0.05, absolute log2 fold change >0.5); these included genes not previously associated with addiction (miR-125a-5p, miR-145 and Foxa1), loci encoding receptors related to drug addiction behaviors and genes with previously recognized roles in addiction such as miR-124, miR-181a, DAT and Ret. Conclusion This study provides insight into the effects of methamphetamine on RNA expression in a key brain region associated with addiction, highlighting the possibility that persistent changes in the expression of genes with both known and previously unknown roles in addiction occur.

Neurological manifestations and molecular genetics of acute intermittent porphyria in North Western Russia

Acute intermittent porphyria (AIP, MIM #176000) is an inherited metabolic disease due to a partial deficiency of the third enzyme, hydroxymethylbilane synthase (HMBS, EC: 4.3.1.8), in the haem biosynthesis. Neurological symptoms during an acute attack, which is the major manifestation of AIP, are variable and relatively rare, but may endanger a patient's life. In the present study, 12 Russian and two Finnish AIP patients with severe neurological manifestations during an acute attack were studied prospectively from 1995 to 2006. Autonomic neuropathy manifested as abdominal pain (88%), tachycardia (94%), hypertension (75%) and constipation (88%). The most common neurological sign was acute motor peripheral neuropathy (PNP, 81%) often associated with neuropathic sensory loss (54%) and CNS involvement (85%). Despite heterogeneity of the neurological manifestations in our patients with acute porphyria, the major pattern of PNP associated with abdominal pain, dysautonomia, CNS involvement and mild hepatopathy could be demonstrated. If more strict inclusion criteria for biochemical abnormalities (>10-fold increase in excretion of urinary PBG) are applied, neurological manifestations in an acute attack are probably more homogeneous than described previously, which suggests that some of the neurological patients described previously may not have acute porphyria but rather secondary porphyrinuria. Screening for acute porphyria using urinary PBG is useful in a selected group of neurological patients with acute PNP or encephalopathy and seizures associated with pain and dysautonomia. Clinical manifestations and the outcome of acute attacks were used as a basis for developing a 30-score scale of the severity of an acute attack. This scale can easily be used in clinical practice and to standardise the outcome of an attack. Degree of muscle weakness scored by MRC, prolonged mechanical ventilation, bulbar paralysis, impairment of consciousness and hyponatraemia were important signs of a poor prognosis. Arrhythmia was less important and autonomic dysfunction, severity of pain and mental symptoms did not affect the outcome. The delay in the diagnosis and repeated administrations of precipitating factors were the main cause of proceeding of an acute attack into pareses and severe CNS involvement and a fatal outcome in two patients. Nerve conduction studies and needle EMG were performed in eleven AIP patients during an acute attack and/or in remission. Nine patients had severe PNP and two patients had an acute encephalopathy but no clinically evident PNP. In addition to axonopathy, features suggestive of demyelination could be demonstrated in patients with severe PNP during an acute attack. PNP with a moderate muscle weakness was mainly pure axonal. Sensory involvement was common in acute PNP and could be subclinical. Decreased conduction velocities with normal amplitudes of evoked potentials during acute attacks with no clinically evident PNP indicated subclinical polyneuropathy. Reversible symmetrical lesions comparable with posterior reversible encephalopathy syndrome (PRES) were revealed in two patients' brain CT or MRI during an acute attack. In other five patients brain MRI during or soon after the symptoms was normal. The frequency of reversible brain oedema in AIP is probably under-estimated since it may be short-lasting and often indistinguishable on CT or MRI. In the present study, nine different mutations were identified in the HMBS gene in 11 unrelated Russian AIP patients from North Western Russia and their 32 relatives. AIP was diagnosed in nine symptom-free relatives. The majority of the mutations were family-specific and confirmed allelic heterogeneity also among Russian AIP patients. Three mutations, c.825+5G>C, c.825+3_825+6del and c.770T>C, were novel. Six mutations, c.77G>A (p.R26H), c.517C>T (p.R173W), c.583C>T (p.R195C), c.673C>T (p.R225X), c.739T>C (p.C247R) and c.748G>C (p.E250A), have previously been identified in AIP patients from Western and other Eastern European populations. The effects of novel mutations were studied by amplification and sequencing of the reverse-transcribed total RNA obtained from the patients' lymphoblastoid or fibroblast cell lines. The mutations c.825+5G>C and c.770T>C resulted in varyable amounts of abnormal transcripts, r.822_825del (p.C275fsX2) and [r.770u>c, r.652_771del, r.613_771del (p.L257P, p.G218_L257del, p.I205_L257del)]. All mutations demonstrated low residual activities (0.1-1.3 %) when expressed in COS-1 cells confirming the causality of the mutations and the enzymatic defect of the disease. The clinical outcome, prognosis and correlation between the HMBS genotype and phenotype were studied in 143 Finnish and Russian AIP patients with ten mutations (c.33G>T, c.97delA, InsAlu333, p.R149X, p.R167W, p.R173W, p.R173Q, p.R225G, p.R225X, c.1073delA) and more than six patients in each group. The patients were selected from the pool of 287 Finnish AIP patients presented in a Finnish Porphyria Register (1966-2003) and 23 Russian AIP patients (diagnosed 1995-2003). Patients with the p.R167W and p.R225G mutations showed lower penetrance (19% and 11%) and the recurrence rate (33% and 0%) in comparison to the patients with other mutations (range 36 to 67% and 0 to 66%, respectively), as well as milder biochemical abnormalities [urinary porphobilinogen 47±10 vs. 163±21 mol/L, p<0.001; uroporphyrin 130±40 vs. 942±183 nmol/L, p<0.001] suggesting a milder form of AIP in these patients. Erythrocyte HMBS activity did not correlate with the porphobilinogen excretion in remission or the clinical of the disease. In all AIP severity patients, normal PBG excretion predicted freedom from acute attacks. Urinary PBG excretion together with gender, age at the time of diagnosis and mutation type could predict the likelihood of acute attacks in AIP patients.

Differential Mechanisms of Binding of Anti-Sigma Factors Escherichia coli Rsd and Bacteriophage T4 AsiA to E. coli RNA Polymerase Lead to Diverse Physiological Consequences

Anti-sigma factors Escherichia coli Rsd and bacteriophage T4 AsiA bind to the essential housekeeping sigma factor, sigma(70), of E. coli. Though both factors are known to interact with the C-terminal region of sigma(70), the physiological consequences of these interactions are very different. This study was undertaken for the purpose of deciphering the mechanisms by which E. coli Rsd and bacteriophage T4 AsiA inhibit or modulate the activity of E. coli RNA polymerase, which leads to the inhibition of E. coli cell growth to different amounts. It was found that AsiA is the more potent inhibitor of in vivo transcription and thus causes higher inhibition of E. coli cell growth. Measurements of affinity constants by surface plasmon resonance experiments showed that Rsd and AsiA bind to or 70 with similar affinity. Data obtained from in vivo and in vitro binding experiments clearly demonstrated that the major difference between AsiA and Rsd is the ability of AsiA to form a stable ternary complex with RNA polymerase. The binding patterns of AsiA and Rsd with sigma(70) studied by using the yeast two-hybrid system revealed that region 4 of sigma(70) is involved in binding to both of these anti-sigma factors; however, Rsd interacts with other regions of sigma(70) as well. Taken together, these results suggest that the higher inhibition of E. coli growth by AsiA expression is probably due to the ability of the AsiA protein to trap the holoenzyme RNA polymerase rather than its higher binding affinity to sigma(70).

RNA triphosphatase and guanylyl transferase activities are associated with the RNA polymerase protein L of rinderpest virus

Rinderpest virus (RPV) large (L) protein is an integral part of the ribonucleoprotein (RNP) complex of the virus that is responsible for transcription and replication of the genome. Previously, we have shown that recombinant L protein coexpressed along with P protein (as the L-P complex) catalyses the synthesis of all viral mRNAs in vitro and the abundance of mRNAs follows a gradient of polarity, similar to the occurrence in vivo. In the present work, we demonstrate that the viral mRNAs synthesized in vitro by the recombinant L or purified RNP are capped and methylated at the N-7 guanine position. RNP from the purified virions, as well as recombinant L protein, shows RNA triphosphatase (RTPase) and guanylyl transferase (GT) activities. L protein present in the RNP complex catalyses the removal of gamma-phosphate from triphosphate-ended 25 nt RNA generated in vitro representing the viral N-terminal mRNA 5' sequence. The L protein forms a covalent enzyme-guanylate intermediate with the GMP moiety of GTP, whose formation is inhibited by the addition of pyrophosphate; thus, it exhibits characteristics of cellular GTs. The covalent bond between the enzyme and nucleotide is acid labile and alkali stable, indicating the presence of phosphoamide linkage. The C-terminal region (aa 1717-2183) of RPV L protein alone exhibits the first step of GT activity needed to form a covalent complex with GMP, though it lacks the ability to transfer GMP to substrate RNA. Here, we describe the biochemical characterization of the newly found RTPase/GT activity of L protein.

Identification and characterization of DdRPB4, a subunit of Dictyostelium discoideum RNA polymerase II

Rpb4, the fourth largest subunit of the eukaryotic RNA polymerase II (RNAPII), is required for growth at extreme temperatures and for an appropriate response to nutrient starvation in yeast. Sequence homologs of Rpb4 are found in most sequenced genomes from yeast to humans. To elucidate the role of this subunit in nutrient starvation, we chose Dictyostelium discoideum, a soil amoeba, which responds to nutrient deprivation by undergoing a complex developmental program. Here we report the identification of homolog of Saccharomyces cerevisiae RPB4 in D. discoideum. Localization and complementation studies suggest that Rpb4 is functionally conserved. DdRPB4 transcript and protein levels are developmentally regulated. Although DdRPB4 could not be deleted, overexpression revealed that the Rpb4 protein is essential for cell survival and is regulated stringently at the post-transcriptional level in D. discoideum. Thus maintaining a critical level of Rpb4 is important for this organism.