The role of SWI4 and SWI6 in the activity of G1 cyclins in yeast.

Entry into the mitotic cycle (START) requires a protein kinase encoded by the CDC28 gene and one of three redundant G1-specific cyclins encoded by CLN1, -2, and -3. SWI4 and SWI6 are transcription factors required for the START-dependent activation of the HO endonuclease gene. They also fulfill an overlapping but essential role for cell division since cells deleted for both genes are inviable. We show that the essential role of SWI4 and SWI6 is to ensure the activity of G1-specific cyclin genes. SWI4 and SWI6 appear necessary for the transcription of CLN1 and CLN2, but not for that of CLN3. CLN3 function is, however, also dependent on SWI4 and SWI6.