TOPICAL REVERSION AT THE HIS1 LOCUS OF SACCHAROMYCES-CEREVISIAE - A TALE OF 3 MUTANTS

Mutants of the HIS1 locus of the yeast Saccharomyces cerevisiae are su itable reporters for spontaneous reversion events because most reversi ons are topical, that is, within the locus itself. Thirteen mutations of his1-1 now have been identified with respect to base sequence. Reve rtants of three mutants and their spontaneous reversion rates are pres ented: (1) a chain termination mutation (his1-208, nee his1-1) that do es not revert by mutations of tRNA loci and reverts only by intracodon ic suppression; (2) a missense mutation (his1-798, nee his1-7) that ca n revert by intragenic suppression by base substitutions of any sort, including a back mutation as well as one three-base deletion; and (3) a -1 frameshift mutation (his1-434, nee his1-19) that only reverts top ically by +1 back mutation, +1 intragenic suppression, or a -2 deletio n. Often the fl insertion is accompanied by base substitution events a t one or both ends of a run of A's. Missense suppressors of his1-798 a re either feeders or nonfeeders, and at four different locations withi n the locus, a single base substitution encoding an amino acid alterat ion will suffice to turn the nonfeeder phenotype into a feeder phenoty pe. Late-appearing revertants of his1-798 were found to be slowly grow ing leaky mutants rather than a manifestation of adaptive mutagenesis. Spontaneous revertants of his1-208 and his1-434 produced no late-aris ing colonies.