Orientation of "plus" genes at the mating-typing locus in homothallic fission yeast.

Results of four crosses are presented which indicate that, in Schizosaccharomyces pombe the plus segment of the mating-typing locus is regulated from a central position, in between of the plus and the minus segment. This conclusion is based on the mapping of a plus-restraining entity r, which is revealed by recombination in certain crosses, or by mutation to unstabe negative strains. To this end, a meiosis-defective mating-type mutation was used as a marker for the plus segment, by taking advantage of its suppressibility by a nonsense suppressor. This suppressibility was substanitated by an additional cross.