Activation of the yeast HO gene by release from multiple negative controls.

Transcription of the yeast HO gene requires five genes, SWI 1, 2, 3, 4, 5. We present evidence that some SWI products activate HO by antagonizing negative regulatory activities encoded by the SIN genes. sin- mutants (defining six genes) were identified because they express HO in the absence of particular SWI products. We argue that SWI5 activates HO by antagonizing SIN3 and that SWI4 activates HO by antagonizing SIN6. HO is expressed in sin3- daughter cells, hence we infer that the SIN3 product represses HO in wild-type daughter cells and that SWI5 and SIN3 are responsible for the cell-lineage-dependent expression of HO. HO is transcribed only when all types of repression are absent: in mother cells, where SWI5 antagonizes SIN3; in late G1, when SWI4 antagonizes SIN6; and in a or alpha cells, where a1-alpha 2 repression is absent.