A NEW PAIR OF B-TYPE CYCLINS FROM SACCHAROMYCES-CEREVISIAE THAT FUNCTION EARLY IN THE CELL-CYCLE

Two new B-type cyclin genes from Saccharomyces cerevisiae, called CLB5 and CLB6, are located in a tail to tail arrangement adjacent to the G 2/M phase promoting cyclins CLB2 and CLB1, respectively. These genomic cyclin arrays are flanked by tRNAs and repeated sequences of Ty eleme nts suggesting an intrachromosomal gene duplication followed by an int erchromosomal gene duplication. Based on their deduced protein sequenc e the CLB5 and CLB6 genes form a new pair of B-type cyclins. They are most related to each other and then to the deduced protein sequence of their adjacent genes CLB1 and CLB2. Both genes are periodically expre ssed, peaking early in the cell cycle. Loss of function mutants are vi able, but clb5- mutants exhibit a delay in S phase whereas clb6- mutan ts show a delay in late G1 and/or S phase. The clb5 mutant phenotype i s somewhat more pronounced in a double null mutant. Both cyclins have the potential to interact with the p34CDC28 kinase in vivo.