CASEIN KINASE-II IS REQUIRED FOR CELL-CYCLE PROGRESSION DURING G(1) AND G(2) M IN SACCHAROMYCES-CEREVISIAE/

The catalytic subunit of Saccharomyces cerevisiae casein kinase II (Sc CKII) is encoded by the CKA1 and CKA2 genes, which together are essen tial for viability. Five independent temperature-sensitive alleles of the CKA2 gene were isolated and used to analyze the function of CHII d uring the cell cycle. Following a shift to the nonpermissive temperatu re, cka2(ts) strains arrested within a single cell cycle and exhibited a dual arrest phenotype consisting of 50% unbudded and 50% large-budd ed cells. The unbudded half of the arrested population contained a sin gle nucleus and a single focus of microtubule staining, consistent wit h arrest in G(1). Most of the large-budded fraction contained segregat ed chromatin and an extended spindle, indicative of arrest in anaphase , though a fraction contained an undivided nucleus with a short thick intranuclear spindle, indicative of arrest in G(2) and/or metaphase. F low cytometry of pheromone-synchronized cells confirmed that CKII is r equired in G(1), at a point which must lie at or beyond Start but prio r to DNA synthesis. Similar analysis of hydroxyurea-synchronized cells indicated that CKII is not required for completion of previously init iated DNA replication but confirmed that the enzyme is again required for cell cycle progression in G(2) and/or mitosis. These results estab lish a role for CKII in regulation and/or execution of the eukaryotic cell cycle.