FIG 1.

Mating-type loci in K. phaffii. (A) Mating-type loci in K. phaffii wild-type cells. The two MAT loci on chromosome 4 are flanked by inverted-repeat regions containing duplicated genes (DIC1 and SLA2). Gene expression from locus 1 (silent locus) is downregulated by telomere position effects, while the genes in locus 2 (active locus) are expressed under mating conditions. The mating type of a cell is determined by the MAT allele in locus 2. Homologous recombination over the DIC1-containing “outer” repeat region leads to mating-type switching by inversion of the region between the two loci. (For further details see reference 22.) (B) Mating-type loci in heterothallic K. phaffii dic1-2Δ cells. Deletion of the DIC1-2-containing region flanking MAT locus 2 prevents mating-type switching.