Abstract
Directionality of yeast mating-type switching has been attributed to differences in chromatin structure for the left arm of chromosome III. We have mapped the structure of approximately 45 kbp of the left arm of chromosome III in a and alpha cells in logarithmically growing cultures and in a cells during switching. Distinctive features of chromatin structure were the occurrence of DNase I-hypersensitive sites in the promoter region of nearly every gene and some replication origins and the presence of extended regions of positioned nucleosomes in approximately 25% of the open reading frames. Other than the recombination enhancer, chromatin structures were identical in the two cell types. Changes in chromatin structure during switching were confined to the recombination enhancer. This unbiased analysis of an extended region of chromatin reveals that significant features of organized chromatin exist for the entire region, and these features are largely static with respect to mating type and mating-type switching. Our analysis also shows that primary chromatin structure does not cause the documented differences in recombinational frequency of the left arm of chromosome III in yeast a and alpha cells.