Epstein C B, Cross F R
Rockefeller University, New York, New York 10021.
Genes Dev. 1992 Sep;6(9):1695-706. doi: 10.1101/gad.6.9.1695.
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.
出芽酵母菌株有三个CLN基因，它们的细胞周期蛋白同源性有限。细胞周期起始至少需要这三个基因中的一个。已知酵母中有四种B型细胞周期蛋白；其中两种已被证明在有丝分裂中起作用。我们发现了第五个B型细胞周期蛋白基因，称为CLB5，当它克隆在CEN质粒上时，可以拯救缺失所有三个CLN基因的菌株。CLB5转录本丰度在G1期达到峰值，与CLN2转录本一致，但早于CLB2转录本。单独缺失CLB5或与其他CLN或CLB缺失联合缺失都不会导致致死性。然而，缺失CLB5的菌株需要更多时间来完成S期，这表明CLB5促进了DNA合成的某个步骤。CLB5是唯一缺失后会延长S期的酵母细胞周期蛋白。CLB5在促进G1/S期转换中可能也有一定作用，因为cln1 cln2菌株在甘油培养基上存活需要CLN3和CLB5，而cln1,2,3-菌株需要CLB5才能被果蝇cdc2基因拯救。结合CLB5过表达对cln1,2,3-的拯救以及CLB5和CLN2的同步转录调控，这些观察结果表明CLB5和CLN基因之间存在部分功能冗余。
