HML is less constrained in MATa cells (top) than in MATα cells (bottom).

The mating type (MAT) locus on Saccharomyces cerevisiae chromosome III can recombine with the HMRa locus near the right telomere or the HMLα locus near the left telomere of the same chromosome. On page 361, Bressan et al. show that the current mating type of a cell determines the spatial configuration of these three loci in the nucleus, suggesting that the outcome of mating type switching is directed by the restraint or release of the left arm of chromosome III.

GFP tagging of chromosomes in the nuclei of living cells showed that movement of the left arm of chromosome III is tightly constrained in MATα cells, but relatively free in MATa cells. Deleting the recombination enhancer (RE) sequence on chromosome III keeps the left arm constrained in both types of cells.

RE activity requires the transcriptional activator Fkh1p, and Bressan et al. suggest that Fkh1p competes for DNA binding with tethering factors that restrain the chromosome. In MATa cells, Fkh1p binding prevails, releasing the left arm and allowing HMLα to recombine with the MAT locus, whereas in MATα cells, restraint of the left arm leaves HMRa as the recombination donor. By directing cells to switch periodically to the opposite mating type, the system assures the availability of mating partners in a haploid population. ▪