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. MAT a preferentially recombines with HML near the left telomere of chromosome III, whereas MATα 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 MATα, 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 MATα and RE-deleted MAT a strains, compared with MAT a. In contrast, chromosome movement at HMR or near a telomere of chromosome V is mating-type independent. HML is more constrained in MAT a Δre and less constrained in MAT a RE(+) compared with other sites. Although HML and MAT a 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 MAT a cells through the action of RE.