Mating type-dependent constraints on the mobility of the left arm of yeast chromosome III.

Mating-type gene (MAT) switching in budding yeast exhibits donor preference. MATa preferentially recombines with HML near the left telomere of chromosome III, whereas MATalpha prefers HMR near the right telomere. Donor preference is controlled by the recombination enhancer (RE) located proximal to HML. To test if HML is constrained in pairing with MATalpha, we examined live-cell mobility of LacI-GFP-bound lactose operator (lacO) arrays inserted at different chromosomal sites. Without induction of recombination, lacO sequences adjacent to HML are strongly constrained in both MATalpha and RE-deleted MATa strains, compared with MATa. In contrast, chromosome movement at HMR or near a telomere of chromosome V is mating-type independent. HML is more constrained in MATa Deltare and less constrained in MATa RE+ compared with other sites. Although HML and MATa are not prealigned before inducing recombination, the three-dimensional configuration of MAT, HML, and HMR is mating-type dependent. These data suggest there is constitutive tethering of HML, which is relieved in MATa cells through the action of RE.