Requirement of the Lec35 Gene for All Known Classes of Monosaccharide-P-Dolichol-dependent Glycosyltransferase Reactions in Mammals

The Lec35 gene product (Lec35p) is required for utilization of the mannose donor mannose-P-dolichol (MPD) in synthesis of both lipid-linked oligosaccharides (LLOs) and glycosylphosphatidylinositols, which are important for functions such as protein folding and membrane anchoring, respectively. The hamster Lec35 gene is shown to encode the previously identified cDNA SL15, which corrects the Lec35 mutant phenotype and predicts a novel endoplasmic reticulum membrane protein. The mutant hamster alleles Lec35.1 and Lec35.2 are characterized, and the human Lec35 gene (mannose-P-dolichol utilization defect 1) was mapped to 17p12-13. To determine whether Lec35p was required only for MPD-dependent mannosylation of LLO and glycosylphosphatidylinositol intermediates, two additional lipid-mediated reactions were investigated: MPD-dependent C-mannosylation of tryptophanyl residues, and glucose-P-dolichol (GPD)-dependent glucosylation of LLO. Both were found to require Lec35p. In addition, the SL15-encoded protein was selective for MPD compared with GPD, suggesting that an additional GPD-selective Lec35 gene product remains to be identified. The predicted amino acid sequence of Lec35p does not suggest an obvious function or mechanism. By testing the water-soluble MPD analog mannose-β-1-P-citronellol in an in vitro system in which the MPD utilization defect was preserved by permeabilization with streptolysin-O, it was determined that Lec35p is not directly required for the enzymatic transfer of mannose from the donor to the acceptor substrate. These results show that Lec35p has an essential role for all known classes of monosaccharide-P-dolichol-dependent reactions in mammals. The in vitro data suggest that Lec35p controls an aspect of MPD orientation in the endoplasmic reticulum membrane that is crucial for its activity as a donor substrate.

Population maintenance among tropical reef fishes: Inferences from small-island endemics

To what extent do local populations of tropical reef fishes persist through the recruitment of pelagic larvae to their natal reef? Endemics from small, isolated islands can help answer that question by indicating whether special biological attributes are needed for long-term survival under enforced localization in high-risk situations. Taxonomically and biologically, the endemics from seven such islands are broadly representative of their regional faunas. As natal-site recruitment occurs among reef fishes in much less isolated situations, these characteristics of island endemics indicate that a wide range of reef fishes could have persistent self-sustaining local populations. Because small islands regularly support substantial reef fish faunas, regional systems of small reserves could preserve much of the diversity of these fishes.

Maintenance of caspase-3 proenzyme dormancy by an intrinsic “safety catch” regulatory tripeptide

Caspase-3 is synthesized as a dormant proenzyme and is maintained in an inactive conformation by an Asp-Asp-Asp “safety-catch” regulatory tripeptide contained within a flexible loop near the large-subunit/small-subunit junction. Removal of this “safety catch” results in substantially enhanced autocatalytic maturation as well as increased vulnerability to proteolytic activation by upstream proteases in the apoptotic pathway such as caspase-9 and granzyme B. The safety catch functions through multiple ionic interactions that are disrupted by acidification, which occurs in the cytosol of cells during the early stages of apoptosis. We propose that the caspase-3 safety catch is a key regulatory checkpoint in the apoptotic cascade that regulates terminal events in the caspase cascade by modulating the triggering of caspase-3 activation.

Requirement of the nicotinic acetylcholine receptor β2 subunit for the anatomical and functional development of the visual system

In the mammalian visual system the formation of eye-specific layers at the thalamic level depends on retinal waves of spontaneous activity, which rely on nicotinic acetylcholine receptor activation. We found that in mutant mice lacking the β2 subunit of the neuronal nicotinic receptor, but not in mice lacking the α4 subunit, retinofugal projections do not segregate into eye-specific areas, both in the dorso-lateral geniculate nucleus and in the superior colliculus. Moreover, β2−/− mice show an expansion of the binocular subfield of the primary visual cortex and a decrease in visual acuity at the cortical level but not in the retina. We conclude that the β2 subunit of the nicotinic acetylcholine receptor is necessary for the anatomical and functional development of the visual system.

In vivo requirement for RecJ, ExoVII, ExoI, and ExoX in methyl-directed mismatch repair

Biochemical studies with model DNA heteroduplexes have implicated RecJ exonuclease, exonuclease VII, exonuclease I, and exonuclease X in Escherichia coli methyl-directed mismatch correction. However, strains deficient in the four exonucleases display only a modest increase in mutation rate, raising questions concerning involvement of these activities in mismatch repair in vivo. The quadruple mutant deficient in the four exonucleases, as well as the triple mutant deficient in RecJ exonuclease, exonuclease VII, and exonuclease I, grow poorly in the presence of the base analogue 2-aminopurine, and exposure to the base analogue results in filament formation, indicative of induction of SOS DNA damage response. The growth defect and filamentation phenotypes associated with 2-aminopurine exposure are effectively suppressed by null mutations in mutH, mutL, mutS, or uvrD/mutU, which encode activities that act upstream of the four exonucleases in the mechanism for the methyl-directed reaction that has been proposed based on in vitro studies. The quadruple exonuclease mutant is also cold-sensitive, having a severe growth defect at 30°C. This phenotype is suppressed by a uvrD/mutU defect, and partially suppressed by mutH, mutL, or mutS mutations. These observations confirm involvement of the four exonucleases in methyl-directed mismatch repair in vivo and suggest that the low mutability of exonuclease-deficient strains is a consequence of under recovery of mutants due to a reduction in viability and/or chromosome loss associated with activation of the mismatch repair system in the absence of RecJ exonuclease, exonuclease VII, exonuclease I, and exonuclease X.

Maintenance of TCR clonality in T cells expressing genes for two TCR heterodimers

T cell receptor (TCR) allelic exclusion is believed to be primarily mediated by suppression of further recombination at the TCR locus after the expression of a functional TCR protein. Genetic allelic exclusion has been shown to be leaky for the β chain and, more commonly, for the α chain. Here, we demonstrate an additional mechanism by which T cells can maintain monoclonality. T cells from double TCR transgenic mice express only one or the other of the two available TCRs at the cell surface. This “functional allelic exclusion” is apparently due to control of the TCR assembly process because these T cells express RNA and protein for all four transgenic TCR proteins. Lack of cell surface expression of the second TCR may be controlled by a failure to assemble the TCR heterodimer.

Regulation of Initiation of S Phase, Replication Checkpoint Signaling, and Maintenance of Mitotic Chromosome Structures during S Phase by Hsk1 Kinase in the Fission Yeast

Hsk1, Saccharomyces cerevisiae Cdc7-related kinase in Shizosaccharomyces pombe, is required for G1/S transition and its kinase activity is controlled by the regulatory subunit Dfp1/Him1. Analyses of a newly isolated temperature-sensitive mutant, hsk1-89, reveal that Hsk1 plays crucial roles in DNA replication checkpoint signaling and maintenance of proper chromatin structures during mitotic S phase through regulating the functions of Rad3 (ATM)-Cds1 and Rad21 (cohesin), respectively, in addition to expected essential roles for initiation of mitotic DNA replication through phosphorylating Cdc19 (Mcm2). Checkpoint defect in hsk1-89 is indicated by accumulation of cut cells at 30°C. hsk1-89 displays synthetic lethality in combination with rad3 deletion, indicating that survival of hsk1-89 depends on Rad3-dependent checkpoint pathway. Cds1 kinase activation, which normally occurs in response to early S phase arrest by nucleotide deprivation, is largely impaired in hsk1-89. Furthermore, Cds1-dependent hyperphosphorylation of Dfp1 in response to hydroxyurea arrest is eliminated in hsk1-89, suggesting that sufficient activation of Hsk1-Dfp1 kinase is required for S phase entry and replication checkpoint signaling. hsk1-89 displays apparent defect in mitosis at 37°C leading to accumulation of cells with near 2C DNA content and with aberrant nuclear structures. These phenotypes are similar to those of rad21-K1 and are significantly enhanced in a hsk1-89 rad21-K1 double mutant. Consistent with essential roles of Rad21 as a component for the cohesin complex, sister chromatid cohesion is partially impaired in hsk1-89, suggesting a possibility that infrequent origin firing of the mutant may affect the cohesin functions during S phase.

The Boron Requirement and Cell Wall Properties of Growing and Stationary Suspension-Cultured Chenopodium album L. Cells1

Suspension-cultured Chenopodium album L. cells are capable of continuous, long-term growth on a boron-deficient medium. Compared with cultures grown with boron, these cultures contained more enlarged and detached cells, had increased turbidity due to the rupture of a small number of cells, and contained cells with an increased cell wall pore size. These characteristics were reversed by the addition of boric acid (≥7 μm) to the boron-deficient cells. C. album cells grown in the presence of 100 μm boric acid entered the stationary phase when they were not subcultured, and remained viable for at least 3 weeks. The transition from the growth phase to the stationary phase was accompanied by a decrease in the wall pore size. Cells grown without boric acid or with 7 μm boric acid were not able to reduce their wall pore size at the transition to the stationary phase. These cells could not be kept viable in the stationary phase, because they continued to expand and died as a result of wall rupture. The addition of 100 μm boric acid prevented wall rupture and the wall pore size was reduced to normal values. We conclude that boron is required to maintain the normal pore structure of the wall matrix and to mechanically stabilize the wall at growth termination.

Transcription from Heterologous rRNA Operon Promoters in Chloroplasts Reveals Requirement for Specific Activating Factors1

The plastid rRNA (rrn) operon in chloroplasts of tobacco (Nicotiana tabacum), maize, and pea is transcribed by the plastid-encoded plastid RNA polymerase from a ς70-type promoter (P1). In contrast, the rrn operon in spinach (Spinacia oleracea) and mustard chloroplasts is transcribed from the distinct Pc promoter, probably also by the plastid-encoded plastid RNA polymerase. Primer-extension analysis reported here indicates that in Arabidopsis both promoters may be active. To understand promoter selection in the plastid rrn operon in the different species, we have tested transcription from the spinach rrn promoter in transplastomic tobacco and from the tobacco rrn promoter in transplastomic Arabidopsis. Our data suggest that transcription of the rrn operon depends on species-specific factors that facilitate transcription initiation by the general transcription machinery.

Oxygen Requirement and Inhibition of C4 Photosynthesis1 : An Analysis of C4 Plants Deficient in the C3 and C4 Cycles

The basis for O2 sensitivity of C4 photosynthesis was evaluated using a C4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate carboxylase-deficient mutant), and a C3-cycle-limited transformant of Flaveria bidentis (an antisense ribulose-1,5-bisphosphate carboxylase/oxygenase [Rubisco] small subunit transformant). Data obtained with the C4-cycle-limited mutant showed that atmospheric levels of O2 (20 kPa) caused increased inhibition of photosynthesis as a result of higher levels of photorespiration. The optimal O2 partial pressure for photosynthesis was reduced from approximately 5 kPa O2 to 1 to 2 kPa O2, becoming similar to that of C3 plants. Therefore, the higher O2 requirement for optimal C4 photosynthesis is specifically associated with the C4 function. With the Rubisco-limited F. bidentis, there was less inhibition of photosynthesis by supraoptimal levels of O2 than in the wild type. When CO2 fixation by Rubisco is limited, an increase in the CO2 concentration in bundle-sheath cells via the C4 cycle may further reduce the oxygenase activity of Rubisco and decrease the inhibition of photosynthesis by high partial pressures of O2 while increasing CO2 leakage and overcycling of the C4 pathway. These results indicate that in C4 plants the investment in the C3 and C4 cycles must be balanced for maximum efficiency.

Binding to the naturally occurring double p53 binding site of the Mdm2 promoter alleviates the requirement for p53 C-terminal activation

Genotoxic stress activation of the tumor suppressor transcription factor p53 involves post-translational C-terminal modifications that increase both protein stability and DNA binding activity. We compared the requirement for p53 protein activation of p53 target sequences in two major p53-regulated genes, p21/WAF1 (encoding a cell cycle inhibitory protein) and Mdm2 (encoding a ubiquitin ligase that targets p53 for proteolytic degradation). The p53 binding site in the proximal p21/WAF1 promoter contains a single p53 binding consensus sequence, while the p53 binding site in the Mdm2 promoter contains two consensus sequences linked by a 17 bp spacer. Binding of recombinant p53 protein to the p21/WAF1 binding site required monoclonal antibody PAb421, which can mimic activating phosphorylation and/or acetylation events at the C-terminus. In contrast, recombinant p53 bound strongly to the Mdm2 binding site in the absence of PAb421 antibody. Separate binding to each consensus sequence of the Mdm2 binding site still required PAb421, indicating that p53 binding was not simply due to greater affinity to the Mdm2 consensus sequences. Linking two p21/WAF1 binding sites with the 17 bp spacer region from the Mdm2 gene eliminated the PAb421 requirement for p53 binding to the p21/WAF1 site. These results suggest a mechanism for regulation of Mdm2 gene transcription that differs from that other p53-induced genes by its lack of a requirement for C-terminal activation of p53 protein. A steady induction of Mdm2 protein would maintain p53 protein at low levels until post-translational modifications following DNA damage increased p53 activity towards other genes, mediating p53 growth inhibitory and apoptotic activities.

Links between replication and recombination in Saccharomyces cerevisiae: A hypersensitive requirement for homologous recombination in the absence of Rad27 activity

The RAD27 gene of Saccharomyces cerevisiae encodes a 5′-3′ flap exo/endonuclease, which plays an important role during DNA replication for Okazaki fragment maturation. Genetic studies have shown that RAD27 is not essential for growth, although rad27Δ mutants are temperature sensitive. Moreover, they exhibit increased sensitivity to alkylating agents, enhanced spontaneous recombination, and repetitive DNA instability. The conditional lethality conferred by the rad27Δ mutation indicates that other nuclease(s) can compensate for the absence of Rad27. Indeed, biochemical and genetical analyses indicate that Okazaki fragment processing can be assured by other enzymatic activities or by alternative pathways such as homologous recombination. Here we present the results of a screen that makes use of a synthetic lethality assay to identify functions required for the survival of rad27Δ strains. Altogether, we confirm that all genes of the Rad52 recombinational repair pathway are required for the survival of rad27Δ strains at both permissive (23°C) and semipermissive (30°C) temperatures for growth. We also find that several point mutations that confer weaker phenotypes in mitotic than in meiotic cells (rad50S, mre11s) and additional gene deletions (com1/sae2, srs2) exhibit synthetic lethality with rad27Δ and that rad59Δ exhibits synergistic effects with rad27Δ. This and previous studies indicate that homologous recombination is the primary, but not only, pathway that functions to bypass the replication defects that arise in the absence of the Rad27 protein.

Rescue of a Wnt mutation by an activated form of LEF-1: Regulation of maintenance but not initiation of Brachyury expression

Members of the LEF-1/TCF family of transcription factors have been implicated in mediating a nuclear response to Wnt signals by association with β-catenin. Consistent with this view, mice carrying mutations in either the Wnt3a gene or in both transcription factor genes Lef1 and Tcf1 were previously found to show a similar defect in the formation of paraxial mesoderm in the gastrulating mouse embryo. In addition, mutations in the Brachyury gene, a direct transcriptional target of LEF-1, were shown to result in mesodermal defects. However, direct evidence for the role of LEF-1 and Brachyury in Wnt3a signaling has been limiting. In this study, we genetically examine the function of LEF-1 in the regulation of Brachyury expression and in signaling by Wnt3a. Analysis of the expression of Brachyury in Lef1−/−Tcf1−/− mice and studies of Brachyury:lacZ transgenes containing wild type or mutated LEF-1 binding sites indicate that Lef1 is dispensable for the initiation, but is required for the maintenance of Brachyury expression. We also show that the expression of an activated form of LEF-1, containing the β-catenin activation domain fused to the amino terminus of LEF-1, can rescue a Wnt3a mutation. Together, these data provide genetic evidence that Lef1 mediates the Wnt3a signal and regulates the stable maintenance of Brachyury expression during gastrulation.

Phosphorylation of MCM4 by cdc2 protein kinase inhibits the activity of the minichromosome maintenance complex.

In eukaryotes, tight regulatory mechanisms ensure the ordered progression through the cell cycle phases. The mechanisms that prevent chromosomal DNA replication from taking place more than once each cell cycle are thought to involve the function of proteins of the minichromosome maintenance (MCM) family. Here, we demonstrate that Xenopus MCM4, a member of the MCM protein family related to Spcdc21/ ScCDC54, is part of a large protein complex comprising several other MCM proteins. MCM4 undergoes cell cycle-dependent phosphorylation both in cleaving embryos and in cell-free extracts. MCM4 phosphorylation starts concomitantly with the clearing of the MCM complex from the chromatin during S phase. Phosphorylation is carried out by cdc2/cyclinB protein kinase, which phosphorylates MCM4 in vitro at identical sites as the ones phosphorylated in vivo. Phosphorylation is specific for cdc2 protein kinase since MCM4 is not a substrate for other members of the cdk family. Furthermore, phosphorylation of MCM4 dramatically reduces its affinity for the chromatin. We propose that the cell cycle-dependent phosphorylation of MCM4 is a mechanism which inactivates the MCM complex from late S phase through mitosis, thus preventing illegitimate DNA replication during that period of the cell cycle.

Cdc45p assembles into a complex with Cdc46p/Mcm5p, is required for minichromosome maintenance, and is essential for chromosomal DNA replication.

We report the isolation and characterization of CDC45, which encodes a polypeptide of 650 amino acids that is essential for the initiation of chromosomal DNA replication in the budding yeast, Saccharomyces cerevisiae. CDC45 genetically interacts with at least two members of the MCM (minichromosome maintenance) family of replication genes, CDC46 and CDC47, which are proposed to perform a role in restricting initiation of DNA replication to once per cell cycle. Like mutants in several MCM genes, alleles of CDC45 also show a severe minichromosome maintenance defect. Together, these observations imply that Cdc45p performs a role in the control of initiation events at chromosomal replication origins. We investigated this possibility further and present evidence demonstrating that Cdc45p is assembled into complexes with one MCM family member, Cdc46p/Mcm5p. These observations point to a role for Cdc45p in controlling the early steps of chromosomal DNA replication in conjunction with MCM polypeptide complexes. Unlike the MCMs, however, the subcellular localization of Cdc45p does not vary with the cell cycle, making it likely that Cdc45p interacts with MCMs only during the nuclear phase of MCM localization in G1.