Sensitivity to the Yeast Plasmid 2mu DNA is conferred by the nuclear allele nibl

Two strains of Saccharomyces carlsbergensis that lacked the plasmid 2mu DNA responded differently when the plasmid was introduced into them. In one strain, cells lacking 2mu DNA ("cir0") produced the normal "smooth" colony morphology, but cells bearing 2mu DNA ("cir+") produced heterogeneous "nibbled" colonies. In the second strain, both cir+ and cir0 strains exhibited a smooth colony morphology. Crosses between these strains revealed that a single recessive nuclear gene, called nibl, conferred the nibbled colony morphology in the presence of 2mu DNA. By a series of backcrosses, nibl was introduced into a Saccharomyces cerevisiae background. nibl caused a nibbled colony morphology in this background just as it did in S. carlsbergensis. nibl was mapped to the left arm of chromosome XVI. Twelve independent smooth revertants were isolated from two nibl [cir+] strains. Seven were analyzed, and all were found to be chromosome VII disomes. Chromosome VII disomy and suppression of the nibbled phenotype cosegregated in crosses. Thus, chromosome VII disomy can suppress the nibbled phenotype. The results of other experiments (C. Holm, Cell 29:585-594, 1982) indicate that the nibbled colony morphology is the result of lethal sectoring and that the lethality is caused by a high copy number of 2mu DNA. I suggest, therefore, that the product of the nibl gene may play a role in controlling the copy number of 2mu DNA. Possible models for the suppression of the nibbled phenotype by chromosome VII disomy are discussed.