CLB5: a novel B cyclin from budding yeast with a role in S phase.

Budding yeast strains have three CLN genes, which have limited cyclin homology. At least one of the three is required for cell cycle START. Four B cyclins are known in yeast; two have been shown to function in mitosis. We have discovered a fifth B-cyclin gene, called CLB5, which when cloned on a CEN plasmid can rescue strains deleted for all three CLN genes. CLB5 transcript abundance peaks in G1, coincident with the CLN2 transcript but earlier than the CLB2 transcript. CLB5 deletion does not cause lethality, either alone or in combination with other CLN or CLB deletions. However, strains deleted for CLB5 require more time to complete S phase, suggesting that CLB5 promotes some step in DNA synthesis. CLB5 is the only yeast cyclin whose deletion lengthens S phase. CLB5 may also have some role in promoting the G1/S transition, because cln1 cln2 strains require both CLN3 and CLB5 for viability on glycerol media and cln1,2,3- strains require CLB5 for rescue by the Drosophila melanogaster cdc2 gene. In conjunction with cln1,2,3- rescue by CLB5 overexpression and the coincident transcriptional regulation of CLB5 and CLN2, these observations are suggestive of partial functional redundancy between CLB5 and CLN genes.