SILENCERS AND DOMAINS OF GENERALIZED REPRESSION

Gene expression can be affected by the chromosomal position of the gen e. An example of this position effect is silencing of the HML and HMR mating-type loci of Saccharomyces cerevisiae. An in vitro assay reveal ed that silencing induced a transcription-independent general occlusio n of the DNA at HMR from sequence-specific interactions of proteins wi th DNA. The minimum boundaries of the silenced chromatin structure wer e determined, as were the contributions of the E and I silencers to th e size of the silenced domain. Examination of endonuclease-sensitive s ites provided evidence that neither the integrity of the chromosomal d uplex nor covalent linkage of the silencers to HMR was important for m aintenance of the silenced structure in vitro.