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

Abstract

Saccharomyces mating-type (MAT) switching occurs by gene conversion using one of two donors, HMLa and HMRa, located near the ends of the same chromosome. MATa cells preferentially choose HMLa, 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 HMLa 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 MATa cells thus reducing its mobility in the nucleus.H APLOID cells of Saccharomyces cerevisiae have the striking ability to change their mating type as often as every generation. Mating-type switching results from a gene conversion event at MAT induced by a double-strand break generated by the site-specific HO endonuclease. One of two loci, HMLa or HMRa, located, respectively, near the left and right ends of chromosome III, is used as a template for the repair event (Haber 2002). Both donors are maintained in heterochromatin and are therefore transcriptionally silent and resistant to HO cleavage (Loo and Rine 1994;Weiss and Simpson 1998;Ravindra et al. 1999). One of the most surprising aspects of mating-type switching is the mating typedependent bias existing in the choice of the donor. MATa cells preferentially (80-90%) recombine with HMLa, whereas 90% of MATa switching cells recombine with HMRa (Klar et al. 1982;Weiler and Broach 1992;Wu and Haber 1995;. Donor preference depends on the activation of the entire left arm of chromosome III, since donors artificially placed at different loci on the left arm are always preferred in MATa cells . When the preferred donor HML is deleted in MATa cells, HMR is easily used instead, meaning that there is no restraint on HMR but instead that the left arm is strongly activated in these cells. However, the deletion of HMR in MATa cells leads to 30% of lethality after HO induction, showing that HML is strongly excluded (Wu and Haber 1995) even though the DNA checkpoint causes the cells to arrest prior to mitosis to have more time to repair the DSB. Therefore, donor preference is mostly controlled by changes on the left arm of chromosome III. The activation of the left arm for recombination is not specific for mating-type switching. The spontaneous rate of recombination between two leu2 alleles, one placed near MA...

Mating-Type Genes andMATSwitching inSaccharomyces cerevisiae

“…Conversely, RE itself can be moved to sites closer to the centromere and still stimulate the use of HML (Coïc et al 2006a). RE can also stimulate HML usage when MAT is moved to a different chromosome (Wu et al 1997).…”

“…First, if a copy of RE is placed near HMR in a MATa strain that also has its normal RE, then the usage of HMR increases from 10-50%, suggesting that RE can activate a nearby HMR in its normal location (Coïc et al 2006a). A second approach was to remove MAT, HML, and HMR from chromosome III and to insert them in roughly the same configuration on the larger chromosome V. Here the use of the more distant HML, on the opposite side of the centromere, was 40%, but when RE was inserted near HML, its use increased to .90% (Coïc et al 2006a). So RE can work in a similar fashion in an entirely different chromosome context.…”

Mating-type Gene Switching in Saccharomyces cerevisiae

“…Conversely, RE itself can be moved to sites along the left arm and still stimulate the use of HML (234). RE can also stimulate HML usage when MAT is moved to a different chromosome (221).…”

Donor Preference Meets Heterochromatin: Moonlighting Activities of a Recombinational Enhancer in Saccharomyces cerevisiae

“…The RE promotes recombination at sites located throughout the entire left arm of chromosome III, which is 110 kbp, and the efficacy of the RE is roughly proportional to its proximity to the locus serving as a donor template Haber 1995, 1996;Coïc et al 2006a). Given that a cells and a cells show similar patterns of DNase-I sensitivity throughout this chromosome arm, the RE does not dramatically alter the local chromatin structure (Ercan and Simpson 2004).…”