Structure and organization of transposable of transposable mating type cassettes in Saccharomyces yeasts.

Abstract

Cell type in Saccharomyces yeasts is regulated by 2 transposable blocks of DNA, the a and .alpha. cassettes. There are 3 loci where either cassette can exist. At the HML and HMR loci the cassettes are not expressed. The cassette at the MAT locus is expressed and controls the cell type. Changes of cell type involve transposition-substitution of cassettes from HML or HMR into MAT. In the molecular cloning of the .alpha. cassette at the HML locus, HML.alpha., there were sequences homologous to HMR and MAT. Using HML.alpha. as a hybridization probe, HMLa, HMR.alpha., HMRa, MAT.alpha. and MATa were isolated. Heteroduplex analysis and restriction endonuclease mapping studies indicate that the a and .alpha. cassettes differ by a substitution corresponding to about 750 base pairs in .alpha. and about 600 base pairs in a. The HML, HMR and MAT loci have regions of homology flanking the position of the a vs. .alpha. substitution. Specific chromosome rearrangements fusing MAT and HML and MAT with HMR were used to orient the cloned sequences on the genetic map; all 3 genes have the same left-to-right polarity on the chromosome.