RESCUE OF LETHAL MUTATIONS IN THE MATING-TYPE REGION OF SCHIZOSACCHAROMYCES-POMBE BY AN EXTRACHROMOSOMAL ELEMENT

In Schizosaccharomyces pombe spontaneous deletions occur in the mating-type (MT) region; the latter comprises closely linked mat loci and intervening L and K regions. The loss of L is lethal; such deletions (e.g., h(+L)) can only be maintained in diploid strains. In tetrads of h(+L)/h(-S) strains we found some viable h(+L) spores yielding colonies with a weak iodine reaction. These wipI clones contain mat2:1-degrees (M) plasmids derived from h(-S). The L region of the plasmid can compensate the chromosomal h(+L) deletion. The episomal M cassette is also expressed. Furthermore, MT switching takes place in this cassette. The plasmids can integrate into the chromosomal MT region giving rise to h90, h(+R) and some other configurations.