Originally published as Genetics Published Articles Ahead of Print on April 19, 2006.

Genetics, Vol. 173, 1197-1206, July 2006, Copyright © 2006
doi:10.1534/genetics.106.055392

Saccharomyces cerevisiae Donor Preference During Mating-Type Switching Is Dependent on Chromosome Architecture and Organization

Eric Coïc1, Guy-Franck Richard2 and James E. Haber3

Department of Biology and Rosenstiel Center, Brandeis University, Waltham, Massachusetts 02254-9110

3 Corresponding author: Department of Biology and Rosenstiel Center, Brandeis University, 415 South St., Waltham, MA 02254-9110.
E-mail: haber{at}brandeis.edu

Saccharomyces mating-type (MAT) switching occurs by gene conversion using one of two donors, HML{alpha} and HMRa, located near the ends of the same chromosome. MATa cells preferentially choose HML{alpha}, a decision that depends on the recombination enhancer (RE) that controls recombination along the left arm of chromosome III (III-L). When RE is inactive, the two chromosome arms constitute separate domains inaccessible to each other; thus HMRa, located on the same arm as MAT, becomes the default donor. Activation of RE increases HML{alpha} usage, even when RE is moved 50 kb closer to the centromere. If MAT is inserted into the same domain as HML, RE plays little or no role in activating HML, thus ruling out any role for RE in remodeling the silent chromatin of HML in regulating donor preference. When the donors MAT and RE are moved to chromosome V, RE increases HML usage, but the inaccessibility of HML without RE apparently depends on other chromosome III-specific sequences. Similar conclusions were reached when RE was placed adjacent to leu2 or arg4 sequences engaged in spontaneous recombination. We propose that RE's targets are anchor sites that tether chromosome III-L in MAT{alpha} cells thus reducing its mobility in the nucleus.