Spontaneous loss of heterozygosity in diploid Saccharomyces cerevisiae cells

To obtain a broad perspective of the events leading to spontaneous loss of heterozygosity (LOH), we have characterized the genetic alterations that fu nctionally inactivated the URA3 marker hemizygously or heterozygously situa ted either on chromosome III or chromosome V in diploid Saccharomyces cerev isiae cells. Analysis of chromosome structure in a large number of LOH clon es by pulsed-field gel electrophoresis and PCR showed that chromosome loss, allelic recombination, and chromosome aberration were the major classes of genetic alterations leading to LOH. The frequencies of chromosome loss and chromosome aberration were significantly affected when the marker was loca ted in different chromosomes, suggesting that chromosome-specific elements may affect the processes that led to these alterations. Aberrant-sized chro mosomes were detected readily in similar to8% of LOH events when the URA3 m arker was placed in chromosome III. Molecular mechanisms underlying the chr omosome aberrations were further investigated by studying the fate of two o ther genetic markers on chromosome III. Chromosome aberration caused by int rachromosomal rearrangements was predominantly due to a deletion between th e MAT and HMR loci that occurred at a frequency of 3.1 x 10(-6). Another rp e of chromosome aberration, which occurred at a frequency slightly higher t han that of the intrachromosomal deletion, appeared to be caused by interch romosomal rearrangement, including unequal crossing over between homologous chromatids and translocation with another chromosome.