The HO gene encodes an endonuclease responsible for initiating mating-type switching, a gene conversion process where MATa cells change to MATalpha cells or vice versa. Cell mating type, MATa or MATalpha is determined by information expressed from the MAT locus. The mating type information is stored in two transcriptionally silenced loci, HMLalpha and HMRa (5,6). Ho initiates switching by recognizing and cleaving a degenerate 24 base-pair site at MAT making a double-stranded break (DSB) in DNA 7. Sequences at MAT are then replaced by copying new sequences from either HML or HMR 3. Although the Ho recognition sequence is present at both HML and HMR, the sequences are occluded by chromatin structure (8). Ho acts in a stoichiometric fashion but appears to act once and then become inactivated (9). Ho degradation by the ubiquitin-26s proteosome system is rapid with a half-life of 10 minutes (10).

HO expression and hence, mating-type interconversion, occurs exclusively in haploid mother cells at the end of G1 after START, the point that marks commitment to duplication events. This timing insures that switching only occurs in G1-arrested cells and that it does not occur during mating or after DNA replication has initiated. To this end, HO transcription is tightly regulated. The promoter of HO lies within a chromosomal region where the chromatin structure is highly regulated. It can be divided into two cis regulatory regions, upstream repression sequence 1 (URS1) and URS2. URS1 (-1000 to -1400) and a TATA-box region (-60 to -90) are required for transcription. URS1 contains binding-sites for the Swi5p transcription factor. URS2 (-150 to -900) deletion renders transcription independent of START. The region contains recognition sites for SBF (Swi4p-Swi6p), a transcription factor that promotes transcription of several late G1 genes (11, 12).

Asymmetric expression of HO in mother cells begins at late anaphase of the cell cycle when Cdc14p dephosphorylates Swi5p. Swi5p then enters mother and daughter nuclei where it binds to and activates several promoters by recruiting SWI/SNF, mediator, and the transcriptional machinery. Though the transcriptional machinery is recruited to the HO promoter late in the cell cycle, HO expression is delayed until the next G1 (13).

In daughter nuclei the repressor Ash1p prevents HO transcription. ASH1 mRNA is asymmetrically distributed to daughter cells where it is translated and accumulates in the daughter nucleus in late anaphase. Ash1p binds to its consensus sequence in the URS1 region of the HO promoter, somehow preventing Swi5p from recruiting transcription factors. At least 20 Ash1p binding sites are present in URS1 (13).

Wild strains of S. cerevisiae express HO and are therefore homothallic. In contrast, the majority of commonly used laboratory strains are ho minus and are therefore heterothallic. In S288C for example, there are four changes in the protein sequence (T189A, G223S, L405S, H475L) (14). Glycine 223 is essential for endonuclease activity and histidine 475 is important for full enzymatic activity (15, 14). In wild strains, the pattern of expression ensures that upon germination of a spore, the immobile yeasts are ultimately near qualified mating partners (16). Hence, most wild S. cerevisiae are diploids. In diploid cells, HO transcription is repressed by the MATa1/MATalpha2 repressor through binding at several MATa1/MATalpha2 binding sites. These binding sites can be used in heterologous promoters to put them under MATa1/MATalpha2 control (12). The HO gene has been used extensively as a reagent to introduce DSBs in the study of recombination and for developing molecular reagents (17).

Last updated: 2005-12-13