Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

Vertebrates have three related Myb genes. The c-Myb protooncogene is required for definitive hematopoiesis in mice and when mutated causes leukemias and lymphomas in birds and mammals. The A-Myb gene is required for spermatogenesis and mammary gland proliferation in mice. The ubiquitously expressed B-Myb gene is essential for early embryonic development in mice and is directly regulated by the p16͞cyclin D͞Rb family͞E2F pathway along with many critical S-phase genes. Drosophila has a single Myb gene most closely related to B-Myb. We have isolated two late-larval lethal alleles of Drosophila Myb. Mutant imaginal discs show an increased number of cells arrested in M phase. Mutant mitotic cells display a variety of abnormalities including spindle defects and increased polyploidy and aneuploidy. Remarkably, some mutant cells have an aberrant S-to M-phase transition in which replicating chromosomes undergo premature histone phosphorylation and chromosomal condensation. These results suggest that the absence of Drosophila Myb causes a defect in S phase that may result in M-phase abnormalities. Consistent with a role for Drosophila Myb during S phase, we detected Dm-Myb protein in S-phase nuclei of wild-type mitotic cells as well as endocycling cells, which lack both an M phase and cyclin B expression. Moreover, we found that the Dm-Myb protein is concentrated in regions of S-phase nuclei that are actively undergoing DNA replication. Together these findings imply that Dm-Myb provides an essential nontranscriptional function during chromosomal replication. The Myb gene family was discovered based on studies of the v-Myb oncogene of the avian myeloblastosis virus that causes leukemia in chickens (1). The normal c-Myb protooncogene is essential for hematopoiesis in mice (2) and when altered causes leukemias and lymphomas in mice and birds. Two additional Myb-related genes are present in vertebrates. The A-Myb gene is essential for spermatogenesis and for mammary gland proliferation in mice (3). Homozygous disruption of B-Myb in the laboratory mouse results in very early lethality at embryonic day 4.5-6.5 (4). These results are consistent with a requirement for B-Myb in cell proliferation that may be masked only briefly by maternal effect during early mouse development. The B-Myb gene is expressed widely throughout mouse development unlike A-Myb or c-Myb (5). The expression of B-Myb seems to correlate with cell division during embryogenesis and adulthood. When quiescent cultured fibroblasts enter the cell cycle, B-Myb mRNA and protein are induced in late G 1 and S phases (6). The B-Myb promoter contains a binding site for the E2F transcription factor that negatively regulates gene expression during G 0 and early G 1 and the Rb family is required for this negative regulation (7-9). Other genes that are similarly regulated include cdc2, cyclin A, thymidylate synthetase, ribonucleotide reductase, and E2F-1 itself. Interestingly, constitutive expression of B-Myb has been reported to bypass p53-induced p21-mediated G 1 arrest (...

We developed a comprehensive dataset that samples the mouse transcriptome every 6 hr, from gastrulation through organogenesis. We observe an abrupt increase in overall transcript diversity at the onset of organogenesis (e8.0); the genes that comprise these changes are preferentially clustered along chromosome 7 and contain a significant enrichment of Gli binding sites. Furthermore, we identify seven dominant patterns of gene expression during gastrulation and organogenesis. Genes clustered according to these seven patterns constitute distinct functional classes, including a cluster enriched for gastrulation and pluripotency genes, two clusters differentially regulating localization and ion metabolism, and three clusters involved in discrete aspects of organogenesis. The last cluster is defined by a dramatic transient decrease in the expression of genes that regulate RNA processing and the cell cycle. Drosophila homologs of these genes are also coordinately downregulated following gastrulation, suggesting that the combined function of these genes has been conserved during metazoan evolution.