Silencers and domains of generalized repression.

Gene expression can be affected by the chromosomal position of the gene. An example of this position effect is silencing of the HML and HMR mating-type loci of Saccharomyces cerevisiae. An in vitro assay revealed that silencing induced a transcription-independent general occlusion of the DNA at HMR from sequence-specific interactions of proteins with DNA. The minimum boundaries of the silenced chromatin structure were determined, as were the contributions of the E and I silencers to the size of the silenced domain. Examination of endonuclease-sensitive sites provided evidence that neither the integrity of the chromosomal duplex nor covalent linkage of the silencers to HMR was important for maintenance of the silenced structure in vitro.