MUTATIONS IN THE PROTEIN PHOSPHATASE-1 GENE AT 87B CAN DIFFERENTIALLYAFFECT SUPPRESSION OF POSITION-EFFECT VARIEGATION AND MITOSIS IN DROSOPHILA-MELANOGASTER

The suppressor of position effect variegation (PEV) locus Su-var(3)6 m aps to 87B5-10. The breakpoints of deficiencies that define this inter val have been placed on a 250-kb molecular map of the region. The locu s is allelic to the ck19 complementation group previously shown to enc ode a type 1 serine-threonine protein phosphatase (PP1) catalytic subu nit. When introduced into flies by P element-mediated transformation, a 5.8-kb genomic fragment carrying this gene overcomes the suppressor phenotype of Su-var(3)6(01) and recessive lethality of all mutations o f the locus. Four of the mutant alleles at the locus show a broad corr elation between high levels of suppression of PEV, a high frequency of aberrant mitosis and low PPI activity in larval extracts. However, so me alleles with low PP1 activity show weak suppression of PEV with a h igh frequency of abnormal mitosis, whereas others show strong suppress ion of PEV with normal mitosis. The basis for these disparate phenotyp es is discussed.