X-RAY AND MEIOTIC FINE STRUCTURE MAPPING OF THE ADENINE-8 LOCUS IN SACCHAROMYCES CEREVISIAE

Abstract

W -H I L E it has been demonstrated in a variety of organisms that genes differ in their spontaneous mutation rates, little is known concerning the basis of these differences. Since the spontaneous mutations affecting adenine biosynthesis in Saccharomyces cerevisiae can be obtained by a selective technique (ROMAN l956), they provide a convenient system in which to study the factors involved in spontaneous mutation rates. In this yeast there are two adenine requiring mutants, adenine-I and adenine-2, which produce a red pigment as a result of the block in adenine synthesis. Haploid cultures of either adenine-I or adenine-2 accumulate white and pink mutants which outgrow the parental red strains. Genetic analysis of these color variants has shown that the interference with pigment production is most often the result of a second mutation in the adenine pathway.The mutants thus obtained are unequally distributed among six loci (ROMAN 1956; JONES 1964). The question of this differential mutability at the adenine loci has been examined by JONES (1 964) in a comparative study of the adenine-3 and adenine-6 loci. The results of her study indicated that there was a positive correlation between the size of these loci, measured by their recombinational lengths and numbers of mutable sites, and the frequency with which they give rise to spontaneous mutations. The twofold difference in the size of these loci, however, was not sufficient to explain the five to tenfold higher mutability of adenine-6 as compared with adenine-3. Further study of the properties of the mutants of each of these loci led to the suggestion that this discrepancy might be due to the failure to recover a substantial fraction of adenine-3 mutants, i.e., that the mutation frequency of adenine-3 is actually higher than is observed.The adenine-8 locus was chosen for investigation in this study because mutants of this gene occur with approximately the same frequency as those of the adenine-3 locus but their properties do not suggest that a large number of mutations are not being recovered. Accordingly adenine-8 should be smaller than adenine-3 and adenine-6, in terms of both recombinational length and number of mutable sites, if gene size is an important component of mutation frequency.In this study the recombinational length of the adenine-8 locus was estimated by two independent methods of allelic mapping. A meiotic fine structure map was constructed using allelic recombination frequencies after meiosis as a meas-1 Supported by Grants 5 R01-AI00328 and 5 TO1-GZI 00328 of the

Recombination and meiosis

“…Holliday 1964a;Zimmermann 1971), as well as between sister chromatids (Perry & Evans 1975). In the case of allelic recombination within a gene, the mitotic frequency can approach that seen in meiosis (Esposito 1968;Holliday, Halliwell, Evans & Rowell 1976). The supposition that this effect is due to the derepression of one or more proteins or enzymes essential for recombination mediated repair is supported by experimental evidence from U. maydis (Holliday 1971(Holliday , 1975Moore 1975).…”

NONSENSE MUTATIONS IN THE ADE3 LOCUS OF SACCHAROMYCES CEREVISIAE

“…70% (39/56) of the mutations at this locus are nonsense mutations; 61% (34/56) are ochre mutations and 9% (5/56) are amber mutations. Previous estimates of ochre mutations among nonselected alleles in yeast have ranged from 33% (HAWTHORNE and MORTIMER 1963;FINK 1966) to 41 % (MANNEY 1964;ESPOSITO 1968), frequencies substantially less than the 61 % reported for the ade3 locus.…”