Identification of a DNA binding factor involved in cell-cycle control of the yeast HO gene.

Transcription of the HO gene is controlled by at least eleven trans-acting regulators, which are responsible for limiting expression to mother cells and to one period in the cell cycle. A subset of these regulators and a repeated element (the cell-cycle box, CCB) are required for expression of HO during late G1. We describe gel retardation and footprinting experiments that identify a factor, CCBF (cell-cycle box factor), that binds to multiple cell-cycle box elements. We show that the SWI4 and SWI6 genes are required for formation of the CCBF-promoter complex in vitro, either as components of CCBF or as modulators of CCBF activity. The other SWI genes (SWI1, 2, 3, and 5) are not required. We also show that SWI4 and SWI6 are the only SWI genes required for expression in vivo from the CCB sequences. These observations indicate that the CCBF is responsible for cell-cycle regulation of HO and lead to the view that two of the SWI genes, SWI4 and SWI6, are specifically involved in this regulatory event.