Figure 6.

2R/3L chimeras at the common left and right flanking regions of the cryptic HMRa and HMLα mating loci were detected through PCR amplification across the left flanking region. (A) Overview of the mating type loci in 2L and 3R. Two blocks of perfect homology that are flanking the cryptic mating loci on the 2R and 3L subtelomeres are shown as dark and light shaded boxes. The left flanking region, or Z region (dark shaded) spans 360 bp, whereas the right flanking region, or X region (light shaded) spans 250 bp. The cryptic mating loci are found in opposite orientations with respect to their telomeres. Enlargement of cryptic mating type loci showing PCR primer locations (arrows) referred to in the text. Using these primers, PCR reactions designed to detect chimera formation across the left and right flanking regions was carried out in strains confirmed to have a terminal 2R deletion. The predicted outcome based on repair by homologous recombination is shown in (B), where recombination across the left flanking Z region results in the formation of a dicentric chromosome and gene conversion of the cryptic mating locus from a to α. The same type of PCR analysis was carried out to detect chimeras formed across the right flanking region, however no positive PCR reactions were identified. The predicted outcome based on homologous recombination across this right flanking region is shown in (C), also resulting in the formation of a dicentric chromosome.