CELL-TYPE-SPECIFIC CHROMATIN ORGANIZATION OF THE REGION THAT GOVERNS DIRECTIONALITY OF YEAST MATING-TYPE SWITCHING

Switching of mating type in Saccharomyces cerevisiae is directional; M AT alpha cells recombine to transfer information from HMRa while MATa cells switch using the silent cassette at HML alpha. Genetic analysis recently has defined a 700 bp recombination enhancer similar to 29 kb from the left end of chromosome III that is necessary for directionali ty. The chromatin structure of this region differs strikingly in a- an d alpha-cells. Mat alpha 2p organizes a 3.7 kb chromatin domain that o pposes interaction of trans-acting proteins with the enhancer, In a-ce lls lacking the alpha 2 repressor, two footprinted regions flank an si milar to 100 bp section having a unique DNA structure, This structural signature probably reflects interactions of proteins that result in d irectional mating type switching.