Ribonucleotide reductase and its regulation during the cell cycle

Ribonucleotide reductase is regulated via the R2 subunit during the life cycle of Trypanosoma brucei

We have examined the occurrence of the R1 and R2 subunits of ribonucleotide reductase during the life cycle of Trypanosoma brucei. Whereas the R1 protein is present throughout the life cycle, the R2 protein is not found in cell cycle-arrested short stumpy trypanosomes. RT-PCR/hybridization analysis revealed almost equal amounts of the R1 and R2 mRNAs in all life cycle stages of the parasite. The data indicate that ribonucleotide reductase of African trypanosomes is developmentally controlled by post-transcriptional regulation of the R2 subunit.

The Caenorhabditis elegans gene ncc-1 encodes a cdc2-related kinase required for M phase in meiotic and mitotic cell divisions, but not for S phase

We have identified six protein kinases that belong to the family of cdc2-related kinases in Caenorhabditis elegans. Results from RNA interference experiments indicate that at least one of these kinases is required for cell-cycle progression during meiosis and mitosis. This kinase, encoded by the ncc-1 gene, is closely related to human Cdk1/Cdc2, Cdk2 and Cdk3 and yeast CDC28/cdc2(+). We addressed whether ncc-1 acts to promote passage through a single transition or multiple transitions in the cell cycle, analogous to Cdks in vertebrates or yeasts, respectively. We isolated five recessive ncc-1 mutations in a genetic screen for mutants that resemble larval arrested ncc-1(RNAi) animals. Our results indicate that maternal ncc-1 product is sufficient for embryogenesis, and that zygotic expression is required for cell divisions during larval development. Cells that form the postembryonic lineages in wild-type animals do not enter mitosis in ncc-1 mutants, as indicated by lack of chromosome condensation and nuclear envelope breakdown. However, progression through G1 and S phase appears unaffected, as revealed by expression of ribonucleotide reductase, incorporation of BrdU and DNA quantitation. Our results indicate that C. elegans uses multiple Cdks to regulate cell-cycle transitions and that ncc-1 is the C. elegans ortholog of Cdk1/Cdc2 in other metazoans, required for M phase in meiotic as well as mitotic cell cycles.

Analysis of a histone H2A variant from fission yeast: evidence for a role in chromosome stability

We have isolated and characterised the pht1 gene from the fission yeast Schizosaccharomyces pombe. The sequence of the predicted translation product has revealed a striking similarity to the family of H2A.F/Z histone variant proteins, which have been found in a variety of different organisms. Cells deleted for the pht1 gene locus grow slowly, exhibit an altered colony morphology, increased resistance to heat shock and show a significant decrease in the fidelity of segregation of an S. pombe minichromosome. We propose that the histone H2A variant encoded by the pht1 gene is important for chromosomal structure and function, possibly including a role in controlling the fidelity of chromosomal segregation during mitosis.

Cell cycle regulation of the Escherichia coli nrd operon: requirement for a cis-acting upstream AT-rich sequence

The expression of the nrd operon encoding ribonucleotide reductase in Escherichia coli has been shown to be cell cycle regulated. To identify the cis-acting elements required for the cell cycle regulation of the nrd promoter, different 5' deletions as well as site-directed mutations were translationally fused to a lacZ reporter gene. The expression of beta-galactosidase from these nrd-lacZ fusions in single-copy plasmids was determined with synchronously growing cultures obtained by repeated phosphate starvation as well as with exponentially growing cultures by flow cytometry analysis. Although Fis and DnaA, two regulatory proteins that bind at multiple sites on the E. coli chromosome, have been found to regulate the nrd promoter, the results in this study demonstrated that neither Fis nor DnaA was required for nrd cell cycle regulation. A cis-acting upstream AT-rich sequence was found to be required for the cell cycle regulation. This sequence could be replaced by a different sequence that maintained the AT richness. A flow cytometry analysis that combined specific immunofluorescent staining of beta-galactosidase with a DNA-specific stain was developed and employed to study the nrd promoter activity in cells at specific cell cycle positions. The results of the flow cytometry analysis confirmed the results obtained from studies with synchronized cells.

Escherichia coli ribonucleotide reductase expression is cell cycle regulated

The expression of the genes encoding ribonucleotide reductase in Escherichia coli was investigated in cultures synchronized by obtaining the smallest cells in a population after sucrose gradient centrifugation. Specific activity of ribonucleotide reductase and DNA initiation were found to increase in parallel, periodically as a function of the cell cycle. The expression of nrd was also determined in cells synchronized by periodic repeated doubling in a phosphate limited medium. Antibodies directed against the B2 subunit of ribonucleotide reductase were raised in a rabbit and purified. Immunoprecipitation of the B2 subunit and RNA-DNA dot blot hybridization assays were developed and employed to determine the expression of ribonucleotide reductase translational and transcriptional products during the cell cycle. Both of nrd-mRNA and B2 subunit expression were found to increase each generation at approximately the same time DNA synthesis was initiated and then to decrease back to the basal level shortly after DNA initiation. These results provided evidence of cell cycle dependent regulation of ribonucleotide reductase in E. coli. When the upstream regulatory region of nrd was fused to a promoterless lacZ gene on a single copy plasmid, lac-mRNA and beta-galactosidase were found to be synthesized in parallel to nrd expression from the chromosomal operon. When nrd sequences surrounding the promoter were removed from this construct, lac-mRNA and beta-galactosidase synthesis were no longer cell cycle regulated.

Control of DNA synthesis genes in fission yeast by the cell-cycle gene cdc10+

In the budding yeast Saccharomyces cerevisiae, cell-cycle control over DNA synthesis occurs partly through the coordinate expression in late G1 phase of many, if not all, of the genes required for DNA synthesis. A cis-acting hexamer element ACGCGT (an MluI restriction site) is responsible for coordinating transcriptional regulation of these genes at the G1/S phase boundary and we have identified a binding activity, DSC1, that recognizes these sequences in a cell-cycle-dependent manner. In the distantly related fission yeast Schizosaccharomyces pombe, only one of the known DNA synthesis genes, cdc22+, which encodes a subunit of ribonucleotide reductase, is periodically expressed in late G1 (ref. 6). The promoter region of cdc22+ has two MluI sites and five related sequences, suggesting that similar controls over DNA synthesis genes could occur in fission yeast. We report here a binding activity in fission yeast that is very similar to DSC1 in budding yeast. We also show that the fission yeast cdc10+ gene product, which is required for Start and entry into S phase, is a component of this binding activity.

A general approach to the isolation of cell cycle-regulated genes in the budding yeast, Saccharomyces cerevisiae

We describe a general approach to the isolation of cell cycle-dependently regulated transcripts in Saccharomyces cerevisiae. This approach is based on the physical identification of cell cycle-regulated transcripts by Northern hybridization using as probes yeast DNA isolated from an ordered S. cerevisiae genomic library. The purpose of this is twofold; first, to assess the importance of transcriptional regulation in cell cycle control; and second, to identify novel genes that may have important roles in the eukaryotic cell cycle. We report the isolation of two previously uncharacterized genes that are transcribed at points in the cell cycle to which specific transcriptional activation has not been assigned: namely, mitosis and early G1 phase. It is argued that these transcripts serve as important landmarks for cell cycle events that are not readily distinguished by either morphological or cytological criteria. The cell cycle-dependent transcription of the RNR1 and CLN1 genes is also described and the implications for cell cycle control, in G1, are discussed with reference to these two genes.