High-resolution structural analysis of chromatin at specific loci: Saccharomyces cerevisiae silent mating-type locus HMRa

Genetic and biochemical evidence implicates chromatin structure in the sile ncing of the two quiescent mating-type loci near the telomeres of chromosom e III in yeast. With high-resolution micrococcal nuclease mapping, we show that the HMRa locus has 12 precisely positioned nucleosomes spanning the di stance between the E and I silencer elements. The nucleosomes are arranged in pairs with very short linkers; the pairs are separated from one another by longer linkers of similar to 20 bp. Both the basic amino-terminal region of histone H4 and the silent information regulator protein Sir3p are neces sary for the organized repressive chromatin structure of the silent locus. Compared to HMRa, only small differences in the availability of the TATA bo x are present for the promoter in the cassette at the active MATa locus. Fe atures of the chromatin structure of this silent locus compared to the prev iously studied HML alpha locus suggest differences in the mechanisms of sil encing and map relate to donor selection during mating-type interconversion .