In mating type switching, donor locus is
In budding yeast, chromosome III harbors two silent donor repositories: HML containing alpha information near left telomere and HMR containing a information near right telomere, each flanked by homology boxes X and Z1 shared with MAT. During switching, MAT is recipient receiving new genetic identity, while HML or HMR acts as template providing sequence without being altered itself, analogous to SDSA. Choice of donor regulated by recombination enhancer controlling accessibility; MATa cells preferentially recombine with HML alpha, MAT alpha cells with HMR a, ensuring productive switching rather
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 21: Silent Loci as Donors for MAT Switching
