SIT4 protein phosphatase is required for the normal accumulation of S WI4, CLN1, CLN2, and HCS26 RNAs during late G₁

In Saccharomyces cerevisiae, the RNA levels of the G₁ cyclins CLN1, CLN2, and HCS26 increase dramatically during the late G₁ phase of the cell cycle. The SIT4 gene, which encodes a serine/threonine protein phosphatase, is required for the normal accumulation of CLN1, CLN2, and HCS26 RNAs during late G₁. This requirement for SIT4 in normal G₁ cyclin RNA accumulation is at least partly via SWI4. Strains containing mutations in SIT4 are sensitive to the loss of either CLN2 or CLN3 function. At the nonpermissive temperature, temperature-sensitive sit4 strains are blocked for both bud emergence and DNA synthesis. Heterologous expression of CLN2 in the absence of SIT4 function results in DNA synthesis, but most of the cells are still blocked for bud emergence. Therefore, SIT4 is required for at least two late G₁ or G₁/S functions: the normal accumulation of G₁ cyclin RNAs (which is required for DNA synthesis) and some additional function that is required for bud emergence or cell cycle progression through late G₁ or G₁/S.