Homothallic haploid cells, carrying an active HO endonuclease gene, can switch to the opposite mating-type allele. When the MAT locus is cleaved by HO endonuclease, the DSB is repaired by homologous recombination (gene conversion) using one of two unexpressed donor loci, HMLa or HMRa, which are located 200 and 100 kb 5' and 3', respectively, on chromosome III. HML and HMR differ from each other in that HML shares more extensive (regions W and Z2) homology with MAT than does HMR. MAT switching displays donor preference, in which MATa selectively recombines with HML whereas MATa preferentially recombines with HMR.
Expression of HO endonuclease is strongly regulated, so that it is turned on only in the late G1 stage of the cell cycle and only in cells that have undergone at least one cell division cycle. After two cell divisions, two MATa cells are juxtaposed to two MATa cells and efficient conjugation produces two MATal MATa zygotes. Coexpression of MATa and MATa turns off the HO gene itself. Thus, once cells switch, the process is complete until meiosis creates haploid segregants.
Much of the research studying HO-mediated MAT switching has relied on freeing the HO gene from its transcriptional controls. Jensen and Herskowitz (1984) fused the HO gene to a galactose-regulated promoter, so that all cells in the population can be induced to switch in a synchronous fashion. The use of the GAL::HO gene has proven to be invaluable in studying not only MAT switching, but in characterizing other forms of DSB repair (NHEJ and break-induced replication) as well as in the detailed study of the DSB-induced DNA damage checkpoint. The applications of HO as a regulated source of defined DSBs to study these different processes have been well reviewed (Moore and Haber 1996a,b; Toczyski et al. 1997; Lee et al. 1998; Nickoloff and Hoekstra 1998; Haber 2000; Melo and Toczyski 2002; Aylon and Kupiec 2004). Here, we focus only on MAT switching itself.
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