Roles of SWI1, SWI2, and SWI3 proteins for transcriptional enhancement by steroid receptors.

The SWI1, SWI2, and SWI3 proteins, which are required for regulated transcription of numerous yeast genes, were found also to be essential for rat glucocorticoid receptor function in yeast; the receptor failed to activate transcription in strains with mutations in the SWI1, SWI2, or SWI3 genes. Certain mutations in genes encoding components of chromatin, identified as suppressors of swi mutations, partially relieved the SWI- requirement for receptor function. Immunoprecipitation of glucocorticoid receptor derivatives from wild-type (SWI+) yeast extracts coprecipitated the SWI3 protein; such receptor-SWI3 complexes were not detected in swi1- or swi2- mutant strains, implying that a complex of multiple SWI proteins may associate with the receptor. Prior incubation of a Drosophila embryo transcription extract with the yeast SWI3-specific antibody inhibited receptor function in vitro whereas the antibody had no effect if added after initiation complex formation. Thus, positive regulation by the glucocorticoid receptor in vivo and in vitro appears to require its interaction, at an early step, with one or more SWI proteins.