SWI-SNF Complex Participation in Transcriptional Activation at a Step Subsequent to Activator Binding

ABSTRACT

The SWI-SNF complex in yeast and related complexes in higher eukaryotes have been implicated in assisting gene activation by overcoming the repressive effects of chromatin. We show that the ability of the transcriptional activator GAL4 to bind to a site in a positioned nucleosome is not appreciably impaired in swimutant yeast cells. However, chromatin remodeling that depends on a transcriptional activation domain shows a considerable, although not complete, SWI-SNF dependence, suggesting that the SWI-SNF complex exerts its major effect at a step subsequent to activator binding. We tested this idea further by comparing the SWI-SNF dependence of a reporter gene based on the GAL10 promoter, which has an accessible upstream activating sequence and a nucleosomal TATA element, with that of a CYC1-lacZ reporter, which has a relatively accessible TATA element. We found that the GAL10-based reporter gene showed a much stronger SWI-SNF dependence than did the CYC1-lacZ reporter with several different activators. Remarkably, transcription of the GAL10-based reporter by a GAL4-GAL11 fusion protein showed a nearly complete requirement for the SWI-SNF complex, strongly suggesting that SWI-SNF is needed to allow access of TFIID or the RNA polymerase II holoenzyme. Taken together, our results demonstrate that chromatin remodeling in vivo can occur by both SWI-SNF-dependent and -independent avenues and suggest that the SWI-SNF complex exerts its major effect in transcriptional activation at a step subsequent to transcriptional activator-promoter recognition.

ACKNOWLEDGMENTS

We thank R. T. Simpson for support and encouragement in the initial phase of this work; S. Hanes, M. Johnston, and D. Picard for gifts of plasmids; C. Peterson for yeast strains, many helpful discussions, and communication of unpublished results; and M. J. Curcio for helpful discussions. We are also grateful to Stephen Johnston for providing a plasmid carrying the MEL1 gene and for help in developing the α-galactosidase assay. We gratefully acknowledge the use of the Wadsworth Center molecular genetics core facility.

This work was supported by NIH grant GM51993 to R.H.M.