Chromosomal rearrangement in Candida stellatoidea results in a positive effect on phenotype.

When type I Candida stellatoidea is plated onto sucrose agar at levels in excess of 10(8) cells, some isolates spontaneously form sucrose-positive colonies. These isolates do not display typical type I phenotypes but instead exhibit phenotypes intermediate between type I C. stellatoidea and C. albicans. Also, this phenotypic change only occurs in conjunction with a chromosomal rearrangement. These rearrangements have been studied in a strain naturally marked for methionine auxotrophy. Chromosome-size DNA bands separated by pulsed-field gel electrophoresis were probed with genes cloned from C. albicans. The hybridization pattern indicated that the genes on several chromosomes underwent extensive rearrangement.

The behavior of insertions near a site of mitotic gene conversion in yeast.

In yeast, coincident gene conversion events involving the LEU1 and TRP5 loci (16 cM apart) occur at frequencies that are far greater than is expected for two independent acts of recombination. When a large plasmid (pJM53) is placed between these genes so that a direct repeat is produced, there is frequent loss of the insert among coincident convertants. Previous results strongly suggest that this is due to a separate, intrachromosomal exchange between the direct repeats rather than to excision from an extensive region of heteroduplex DNA. In this paper, we extend our genetic and molecular analysis to a plasmid insertion (pKSH) which replaces rather than duplicates the chromosomal material. The relative stabilities of pKSH and pJM53 are compared among coincident Leu+Trp+ convertants and convertants involving only one locus (LEU1). The pKSH insertion is significantly more stable in the latter which constitute a large majority of the selectable recombinants. In the former, both insertions are lost with high frequency. These results are used to argue that, while most mitotic conversion does not result from long intermediates, coincident convertants may arise from either multiple intermediates or extensive heteroduplex regions.

Coincident recombination during mitosis in saccharomyces: distance-dependent and -independent components.

In mitosis, coincident recombination events between widely separated markers occur more frequently than expected for two independent acts. Several different mechanisms have been proposed to account for this phenomenon. It has been argued that coincident recombination could be due to either an extensive region of heteroduplex DNA or some other distance-dependent mechanism. Alternately, it has been suggested that at least some is due to subpopulations of cells which undergo recombination at very high frequencies. The purpose of these experiments is to evaluate the possible contribution of distance-dependent and distance-independent components. By comparing the coincident recombination frequencies for markers on the same homolog as well as pairs of unlinked sites, we show that there is a strong distance-dependent component for at least 8.8-35-kbp, depending on the type of recombination event (conversion or intrachromosomal exchange). For larger distances separating sites, a distance-independent mechanism(s) results in higher than expected frequencies.

Coincident gene conversion events in yeast that involve a large insertion.

In yeast, spontaneous gene conversion events involving sites that are far apart (16 cM) occur 1000 times more frequently in mitotic cells than is expected for two independent acts of recombination. It has been proposed that a major portion of these could be due to a long, continuous heteroduplex intermediate. We have examined this possibility in further detail by introducing, via transformation, a large plasmid insertion between the LEU1 and TRP5 loci and studying its behavior among coincident convertants involving the flanking sites. Among such convertants, there is frequent loss of the plasmid when it is present in hemizygous or homozygous configuration. Our results could support the long heteroduplex model for coincident recombination events, but only if novel assumptions regarding the formation and fate of mismatched DNA are made. Therefore, an alternative model that proposes multiple, concerted recombination events is discussed.

Coincident gene conversion during mitosis in saccharomyces.

During mitosis, gene conversion events at the TRP5 locus on chromosome VII are coupled with conversion events at LEU1, a locus 18 cM away, 1200 times more frequently than would be expected for two independent acts of recombination. Such coincident conversion events that occur over relatively long distances could be due to several mechanisms. We discuss these possibilities and describe an experiment that indicates that a portion of coincident events is due to extensive heteroduplexes. The phenomenon of coincident gene conversion is discussed in relation to our earlier evidence that spontaneous recombination between homologues occurs prereplicationally in mitosis.

Mitotic recombination: mismatch correction and replicational resolution of Holliday structures formed at the two strand stage in Saccharomyces.

In a preliminary report (Esposito 1978), evidence was presented which showed that heteroallelic recombination resulting in prototrophic colonies occurs at the 2-strand stage. A model utilizing replicative resolution of Holliday structures was proposed to explain how gene conversion at the 2-strand stage can result in exchange of outside markers. The object of the experiments reported herein was to present detailed genetic evidence for 2-strand recombination. In addition, we examined the features of mitotic recombination with respect to symmetry, length and polarity of heteroduplexes in wild type strains (REM1/REM1) and in strains bearing the hyper-recombination mutation rem1-1. To do this, we constructed strains so that prototrophs arising from heteroallelic recombination and recombinant for outside markers were detected by visual inspection. By analyzing these colonies genetically, we have inferred several features of mitotic recombination which distinguish it from its meiotic counterpart. Firstly, mitotic heteroduplexes are often symmetric while meiotic heteroduplexes are almost exclusively asymmetric. Secondly, heteroduplexes tend to be longer in mitosis that in meiosis. Thirdly, unlike meiotic conversion, mitotic conversion does not show strong polarity. Recombination in strains homozygous for the rem1-1 mutation also takes place at the 2-strand stage. The rem1-1 mutation, however, appears to alter the features of mismatch correction.

Mitotic versus meiotic recombination in Saccharomyces cerevisiae.

As part of a comparative analysis of spontaneous mitotic and meiotic recombination we have compared the mitotic and meiotic maps of the wild type and yeast hybrids homozygous for reml-l, a mitosis-specific hyper-rec mutation (Golin and Esposito, 1977; Golin, 1979). In wild type yeast strains recombination in centromere proximal intervals occurs relatively more frequently in mitosis than in meiosis. In reml-1/rem1-1 hybrids the distribution of mitotic exchange events is more similar to the distribution observed in meiosis.

Evidence for joint genic control of spontaneous mutation and genetic recombination during mitosis in Saccharomyces.

A procedure for detection of mutants exhibiting either enhanced or reduced spontaneous mutation during mitosis and/or meiosis has been developed to probe the joint genic control of spontaneous mutation and recombination in yeast. A semidominant mutator, rem1-1, recovered by this technique, exhibits enhanced spontaneous mutation,intragenic recombination, and intergenic recombination during mitosis. Diploids homozygous for rem1-1 exhibit normal levels of meiotic intragenic and intergenic recombination and diminished ascospore viability.