Clb5-associated Kinase Activity is Required Early in the Spindle Pathway for Correct Preanaphase Nuclear Positioning in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, a single cyclin-dependent kinase, Cdc28, regulates both G1/S and G2/M phase transitions by associating with stage-specific cyclins. During progression through S phase and G2/M, Cdc28 is activated by the B-type cyclins Clb1–6. Because of functional redundancy, specific roles for individual Clbs have been difficult to assign. To help genetically define such roles, strains carrying a cdc28^ts allele, combined with single CLB deletions were studied. We assumed that by limiting the activity of the kinase, these strains would be rendered more sensitive to loss of individual Clbs.