Evolution of a quadripartite hybrid virus by interspecific exchange and recombination between replicase components of two related tripartite RNA viruses

Cucumber mosaic virus (CMV) and tomato aspermy virus (TAV) belong to the Cucumovirus genus. They have a tripartite genome consisting of single-stranded RNAs, designated 1, 2, and 3. Previous studies have shown that viable pseudorecombinants could be created in vitro by reciprocal exchanges between CMV and TAV RNA 3, but exchanges of RNAs 1 and 2 were replication deficient. When we coinoculated CMV RNAs 2 and 3 along with TAV RNAs 1 and 2 onto Nicotiana benthamiana, a hybrid quadripartite virus appeared that consisted of TAV RNA 1, CMV RNAs 2 and 3, and a distinctive chimeric RNA originating from a recombination between CMV RNA 2 and the 3′-terminal 320 nucleotides of TAV RNA 2. This hybrid arose by means of segment reassortment and RNA recombination to produce an interspecific hybrid with the TAV helicase subunit and the CMV polymerase subunit. To our knowledge, this is the first report demonstrating the evolution of a new plant or animal virus strain containing an interspecific hybrid replicase complex.

Formation of the preprimosome protects λ O from RNA transcription-dependent proteolysis by ClpP/ClpX

Using the bacteriophage λ DNA replication system, composed entirely of purified proteins, we have tested the accessibility of the short-lived λ O protein to the ClpP/ClpX protease during the various stages of λ DNA replication. We find that binding of λ O protein to its oriλ DNA sequence, leading to the so-called “O-some” formation, largely inhibits its degradation. On the contrary, under conditions permissive for transcription, the λ O protein bound to the oriλ sequence becomes largely accessible to ClpP/ClpX-mediated proteolysis. However, when the λ O protein is part of the larger oriλ:O⋅P⋅DnaB preprimosomal complex, transcription does not significantly increase ClpP/ClpX-dependent λ O degradation. These results show that transcription can stimulate proteolysis of a protein that is required for the initiation of DNA replication.

RNA polymerase subunit RPB5 plays a role in transcriptional activation

A mutation in RPB5 (rpb5–9), an essential RNA polymerase subunit assembled into RNA polymerases I, II, and III, revealed a role for this subunit in transcriptional activation. Activation by GAL4-VP16 was impaired upon in vitro transcription with mutant whole-cell extracts. In vivo experiments using inducible reporter plasmids and Northern analysis support the in vitro data and demonstrate that RPB5 influences activation at some, but not all, promoters. Remarkably, this mutation maps to a conserved region of human RPB5 implicated by others to play a role in activation. Chimeric human-yeast RPB5 containing this conserved region now can function in place of its yeast counterpart. The defects noted with rpb5–9 are similar to those seen in truncation mutants of the RPB1-carboxyl terminal domain (CTD). We demonstrate that RPB5 and the RPB1-CTD have overlapping roles in activation because the double mutant is synthetically lethal and has exacerbated activation defects at the GAL1/10 promoter. These studies demonstrate that there are multiple activation targets in RNA polymerase II and that RPB5 and the CTD have similar roles in activation.

RNA as a target of double-stranded RNA-mediated genetic interference in Caenorhabditis elegans

Introduction of exogenous double-stranded RNA (dsRNA) into Caenorhabditis elegans has been shown to specifically and potently disrupt the activity of genes containing homologous sequences. In this study we present evidence that the primary interference effects of dsRNA are post-transcriptional. First, we examined the primary DNA sequence after dsRNA-mediated interference and found no evidence for alterations. Second, we found that dsRNA-mediated interference with the upstream gene in a polar operon had no effect on the activity of the downstream gene; this finding argues against an effect on initiation or elongation of transcription. Third, we observed by in situ hybridization that dsRNA-mediated interference produced a substantial, although not complete, reduction in accumulation of nascent transcripts in the nucleus, while cytoplasmic accumulation of transcripts was virtually eliminated. These results indicate that the endogenous mRNA is the target for interference and suggest a mechanism that degrades the targeted RNA before translation can occur. This mechanism is not dependent on the SMG system, an mRNA surveillance system in C. elegans responsible for targeting and destroying aberrant messages. We suggest a model of how dsRNA might function in a catalytic mechanism to target homologous mRNAs for degradation.

Histones H3 and H4 are components of upstream activation factor required for the high-level transcription of yeast rDNA by RNA polymerase I

RNA polymerase I (Pol I) transcription in the yeast Saccharomyces cerevisiae is greatly stimulated in vivo and in vitro by the multiprotein complex, upstream activation factor (UAF). UAF binds tightly to the upstream element of the rDNA promoter, such that once bound (in vitro), UAF does not readily exchange onto a competing template. Of the polypeptides previously identified in purified UAF, three are encoded by genes required for Pol I transcription in vivo: RRN5, RRN9, and RRN10. Two others, p30 and p18, have remained uncharacterized. We report here that the N-terminal amino acid sequence, its mobility in gel electrophoresis, and the immunoreactivity of p18 shows that it is histone H3. In addition, histone H4 was found in UAF, and myc-tagged histone H4 could be used to affinity-purify UAF. Histones H2A and H2B were not detectable in UAF. These results suggest that histones H3 and H4 probably account for the strong binding of UAF to DNA and may offer a means by which general nuclear regulatory signals could be transmitted to Pol I.

A mutant RNA polymerase that forms unusual open promoter complexes

We describe a mutant Escherichia coli RNA polymerase (RNAP) that forms stable open promoter complexes even at −20°C but with a shortened melted region that extends downstream to only position −7. In the presence of initiating transcription substrates, the mutant RNAP undergoes a temperature-dependent isomerization, resulting in a promoter complex that is indistinguishable from the wild-type RNAP–promoter complex, with the melted region extended downstream to position +4. We propose that the open complex formed by the mutant RNAP represents an intermediate on the normal promoter-opening pathway and that our results support earlier findings that initial promoter opening occurs in the upstream region of the −10 promoter consensus element and subsequently extends downstream to encompass the transcription start site.

The nucleocapsid protein specifically anneals tRNALys-3 onto a noncomplementary primer binding site within the HIV-1 RNA genome in vitro

HIV type 1 (HIV-1) specifically uses host cell tRNALys-3 as a primer for reverse transcription. The 3′ 18 nucleotides of this tRNA are complementary to a region on the HIV RNA genome known as the primer binding site (PBS). HIV-1 has a strong preference for maintaining a lysine-specific PBS in vivo, and viral genomes with mutated PBS sequences quickly revert to be complementary to tRNALys-3. To investigate the mechanism for the observed PBS reversion events in vitro, we examined the capability of the nucleocapsid protein (NC) to anneal various tRNA primer sequences onto either complementary or noncomplementary PBSs. We show that NC can anneal different full-length tRNAs onto viral RNA transcripts derived from the HIV-1 MAL or HXB2 isolates, provided that the PBS is complementary to the tRNA used. In contrast, NC promotes specific annealing of only tRNALys-3 onto an RNA template (HXB2) whose PBS sequence has been mutated to be complementary to the 3′ 18 nt of human tRNAPro. Moreover, HIV-1 reverse transcriptase extends this binary complex from the proline-specific PBS. The formation of the noncomplementary binary complex does not occur when a chimeric tRNALys/Pro containing proline-specific D and anticodon domains is used as the primer. Thus, elements outside the acceptor-TΨC domains of tRNALys-3 play an important role in preferential primer use in vitro. Our results support the hypothesis that mutant PBS reversion is a result of tRNALys-3 annealing onto and extension from a PBS that specifies an alternate host cell tRNA.

Escherichia coli RNA polymerase terminates transcription efficiently at rho-independent terminators on single-stranded DNA templates

Several models have been proposed for the mechanism of transcript termination by Escherichia coli RNA polymerase at rho-independent terminators. Yager and von Hippel (Yager, T. D. & von Hippel, P. H. (1991) Biochemistry 30, 1097–118) postulated that the transcription complex is stabilized by enzyme–nucleic acid interactions and the favorable free energy of a 12-bp RNA–DNA hybrid but is destabilized by the free energy required to maintain an extended transcription bubble. Termination, by their model, is viewed simply as displacement of the RNA transcript from the hybrid helix by reformation of the DNA helix. We have proposed an alternative model where the RNA transcript is stably bound to RNA polymerase primarily through interactions with two single-strand specific RNA-binding sites; termination is triggered by formation of an RNA hairpin that reduces binding of the RNA to one RNA-binding site and, ultimately, leads to its ejection from the complex. To distinguish between these models, we have tested whether E. coli RNA polymerase can terminate transcription at rho-independent terminators on single-stranded DNA. RNA polymerase cannot form a transcription bubble on these templates; thus, the Yager–von Hippel model predicts that intrinsic termination will not occur. We find that transcript elongation on single-stranded DNA templates is hindered somewhat by DNA secondary structure. However, E. coli RNA polymerase efficiently terminates and releases transcripts at several rho-independent terminators on such templates at the same positions as termination occurs on duplex DNAs. Therefore, neither the nontranscribed DNA strand nor the transcription bubble is essential for rho-independent termination by E. coli RNA polymerase.

Yeast mutations in multiple complementation groups inhibit brome mosaic virus RNA replication and transcription and perturb regulated expression of the viral polymerase-like gene

Brome mosaic virus (BMV), a member of the alphavirus-like superfamily of positive-strand RNA viruses, encodes two proteins, 1a and 2a, that interact with each other, with unidentified host proteins, and with host membranes to form the viral RNA replication complex. Yeast expressing 1a and 2a support replication and subgenomic mRNA synthesis by BMV RNA3 derivatives. Using a multistep selection and screening process, we have isolated yeast mutants in multiple complementation groups that inhibit BMV-directed gene expression. Three complementation groups, represented by mutants mab1–1, mab2–1, and mab3–1 (for maintenance of BMV functions), were selected for initial study. Each of these mutants has a single, recessive, chromosomal mutation that inhibits accumulation of positive- and negative-strand RNA3 and subgenomic mRNA. BMV-directed gene expression was inhibited when the RNA replication template was introduced by in vivo transcription from DNA or by transfection of yeast with in vitro transcripts, confirming that cytoplasmic RNA replication steps were defective. mab1–1, mab2–1, and mab3–1 slowed yeast growth to varying degrees and were temperature-sensitive, showing that the affected genes contribute to normal cell growth. In wild-type yeast, expression of the helicase-like 1a protein increased the accumulation of 2a mRNA and the polymerase-like 2a protein, revealing a new level of viral regulation. In association with their other effects, mab1–1 and mab2–1 blocked the ability of 1a to stimulate 2a mRNA and protein accumulation, whereas mab3–1 had elevated 2a protein accumulation. Together, these results show that BMV RNA replication in yeast depends on multiple host genes, some of which directly or indirectly affect the regulated expression and accumulation of 2a.

Widespread expression and estrogen regulation of PPEIA-3′ nuclear RNA in the rat brain

We previously identified a novel nuclear RNA species derived from the preproenkephalin (PPE) gene. This transcript, which we have named PPEIA-3′ RNA, hybridizes with probes directed at a region of PPE intron A downstream of an alternative germ-cell transcription start site, but does not contain PPE protein coding sequences. We now report that estrogen treatment of ovariectomized rats increases the expression of conventional PPE heteronuclear RNA, and also induces the expression of PPEIA-3′ RNA, apparently in separate cell populations within the ventromedial nucleus of the hypothalamus. Further, we show that cells expressing PPEIA-3′ are found in several neuronal groups in the rat forebrain and brainstem, with a distinct topographical distribution. High densities of PPEIA-3′ containing cells are found in the reticular thalamic nucleus, the basal forebrain, the vestibular complex, the deep cerebellar nuclei, and the trapezoid body, a pattern that parallels the distribution of atypical nuclear RNAs described by other groups. These results suggest that this diverse neuronal population shares a common set of nuclear factors responsible for the expression and retention of this atypical RNA transcript. The implication of these results for cell-specific gene transcription and regulation in the brain and the possible relationship of PPEIA-3′ RNA and other atypical nuclear RNAs is discussed.

A zinc finger-containing papain-like protease couples subgenomic mRNA synthesis to genome translation in a positive-stranded RNA virus

The genome expression of positive-stranded RNA viruses starts with translation rather than transcription. For some viruses, the genome is the only viral mRNA and expression is regulated primarily at the translational level and by limited proteolysis of polyproteins. Other virus groups also generate subgenomic mRNAs later in the reproductive cycle. For nidoviruses, subgenomic mRNA synthesis (transcription) is discontinuous and yields a 5′ and 3′ coterminal nested set of mRNAs. Nidovirus transcription is not essential for genome replication, which relies on the autoprocessing products of two replicase polyproteins that are translated from the genome. We now show that the N-terminal replicase subunit, nonstructural protein 1 (nsp1), of the nidovirus equine arteritis virus is in fact dispensable for replication but crucial for transcription, thereby coupling replicase expression and subgenomic mRNA synthesis in an unprecedented manner. Nsp1 is composed of two papain-like protease domains and a predicted N-terminal zinc finger, which was implicated in transcription by site-directed mutagenesis. The structural integrity of nsp1 is essential, suggesting that the protease domains form a platform for the zinc finger to operate in transcription.

TectoRNA: modular assembly units for the construction of RNA nano-objects

Structural information on complex biological RNA molecules can be exploited to design tectoRNAs or artificial modular RNA units that can self-assemble through tertiary interactions thereby forming nanoscale RNA objects. The selective interactions of hairpin tetraloops with their receptors can be used to mediate tectoRNA assembly. Here we report on the modulation of the specificity and the strength of tectoRNA assembly (in the nanomolar to micromolar range) by variation of the length of the RNA subunits, the nature of their interacting motifs and the degree of flexibility of linker regions incorporated into the molecules. The association is also dependent on the concentration of magnesium. Monitoring of tectoRNA assembly by lead(II) cleavage protection indicates that some degree of structural flexibility is required for optimal binding. With tectoRNAs one can compare the binding affinities of different tertiary motifs and quantify the strength of individual interactions. Furthermore, in analogy to the synthons used in organic chemistry to synthesize more complex organic compounds, tectoRNAs form the basic assembly units for constructing complex RNA structures on the nanometer scale. Thus, tectoRNA provides a means for constructing molecular scaffoldings that organize functional modules in three-dimensional space for a wide range of applications.

RNA-templated DNA ligation for transcript analysis

Ligase-mediated gene detection has proven valuable for detection and precise distinction of DNA sequence variants. We have recently shown that T4 DNA ligase can also be used to distinguish single nucleotide variants of RNA sequences. Here we describe parameters that influence RNA-templated DNA ligation by T4 DNA ligase. The reaction proceeds much more slowly, requiring more enzyme, compared to ligation of the same oligonucleotides hybridized to the corresponding DNA sequence. The reaction is inhibited at high concentrations of ATP and NaCl and both magnesium and manganese ions can support the reaction. We define reaction conditions where 80% of RNA target molecules can template a diagnostic ligation reaction. Ligase-mediated RNA detection should provide a useful mechanism for sensitive and accurate detection and distinction of RNA sequence variants.

The transcript release factor PTRF augments ribosomal gene transcription by facilitating reinitiation of RNA polymerase I

Termination of murine rDNA transcription by RNA polymerase I (Pol I) requires pausing of Pol I by terminator-bound TTF-I (transcription termination factor for Pol I), followed by dissociation of the ternary complex by PTRF (Pol I and transcript release factor). To examine the functional correlation between transcription termination and initiation, we have compared transcription on terminator-containing and terminator-less rDNA templates. We demonstrate that terminated RNA molecules are more efficiently synthesized than run-off transcripts, indicating that termination facilitates reinitiation. Transcriptional enhancement is observed in multiple- but not single-round transcription assays measuring either promoter-dependent or promoter-independent Pol I transcription. Increased synthesis of terminated transcripts is observed in crude extracts but not in a PTRF-free reconstituted transcription system, indicating that PTRF-mediated release of pre-rRNA is responsible for transcriptional enhancement. Consistent with PTRF serving an important role in modulating the efficiency of rRNA synthesis, PTRF exhibits pronounced charge heterogeneity, is phosphorylated at multiple sites and fractionates into transcriptionally active and inactive forms. The results suggest that regulation of PTRF activity may be an as yet unrecognized means to control the efficiency of ribosomal RNA synthesis.