Testing cyclin specificity in the exit from mitosis

Cyclical inactivation of B-type cyclins has been proposed to be required fo r alternating DNA replication and mitosis, Destruction box-dependent Clb5p degradation is strongly increased in mitotic cells, and constitutive overex pression of Clb5p lacking the destruction box resulted in rapid accumulatio n of inviable cells, frequently multiply budded, with DNA contents ranging from unreplicated to apparently fully replicated. Loss of viability correla ted with retention of nuclear Clb5p at the time of nuclear division. CLB2-D elta db overexpression that was quantitatively comparable to CLB5-Delta db overexpression with respect to Clb protein production and Clb-associated ki nase activity resulted in a distinct phenotype: reversible mitotic arrest w ith uniformly replicated DNA. Simultaneous overexpression of CLB2-Delta db and CLB5-Delta db overexpressers similarly resulted in a uniform arrest wit h replicated DNA, and this arrest was significantly more reversible than th at observed with CLB5-Delta db overexpression alone. These results suggest that Clb2p and not Clb5p can efficiently block mitotic completion. We specu late that CLB5-Delta db overexpression may be lethal, because persistence o f high nuclear Clb5p-associated kinase throughout mitosis leads to failure to load origins of replication, thus preventing DNA replication in the succ eeding cell cycle.