title: Spontaneous mutagenesis in haploid and diploid Saccharomyces cerevisiae.
abstract: To obtain insights into the mechanisms of spontaneous mutations in Saccharomyces cerevisiae, we have characterized the genetic alterations that inactivate either the CAN1 gene in haploid cells or heterozygously situated in diploid cells. The mutation rate in haploid cells was 9.08 x 10(-7), 100-fold lower than that in diploid cells (1.03 x 10(-4)). In haploid cells, among 69 independent CAN1 mutations, 75% were base substitutions and 22% frameshifts. The base substitutions were both transitions (33%) and transversions (42%), with G:C-->A:T and G:C-->T:A dominating. Minus frameshifts (12%) and plus frameshifts (10%) were also observed at run and non-run bases, and at A:T and G:C pairs with almost equal efficiency. An analysis of chromosome structure in diploid yeast cells indicated that allelic crossover was the predominant event followed by gene conversion and chromosome loss. We argued that genetic alterations leading to spontaneous phenotypic changes in wild-type diploid yeast cells occurred through two steps; replication-dependent alterations of bases in either allele then recombination-dependent transfer of the mutated allele to the intact one.
authors.affiliation: Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
authors.forename: Gaku
authors.initials: G
authors.lastname: Ohnishi
authors.forename: Kingo
authors.initials: K
authors.lastname: Endo
authors.forename: Akiko
authors.initials: A
authors.lastname: Doi
authors.forename: Atsushige
authors.initials: A
authors.lastname: Fujita
authors.forename: Yasukazu
authors.initials: Y
authors.lastname: Daigaku
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Nunoshiba
authors.forename: Kazuo
authors.initials: K
authors.lastname: Yamamoto
mesh_terms: D026905:Amino Acid Transport Systems / Q000235:genetics*; D002874:Chromosome Mapping / Q000379:methods; D015825:Chromosomes, Fungal / Q000235:genetics*; D004252:DNA Mutational Analysis / Q000379:methods; D004171:Diploidy; D005656:Fungal Proteins / Q000235:genetics*; D015966:Gene Expression Regulation, Fungal / Q000235:genetics; D005787:Gene Frequency; D014644:Genetic Variation / Q000235:genetics; D006238:Haploidy; D016296:Mutagenesis / Q000235:genetics*; D012441:Saccharomyces cerevisiae / Q000235:genetics*
chemical_list: D026905:Amino Acid Transport Systems; C097640:CAN1 protein, Candida albicans; D005656:Fungal Proteins