HO/YDL227C Summary Help

Standard Name HO 1
Systematic Name YDL227C
Feature Type ORF, Verified
Description Site-specific endonuclease; required for gene conversion at the MAT locus (homothallic switching) through the generation of a ds DNA break; expression restricted to mother cells in late G1 as controlled by Swi4p-Swi6p, Swi5p, and Ash1p (2, 3, 4, 5 and see Summary Paragraph)
Name Description HOmothallic switching endonuclease 3, 5
Chromosomal Location
ChrIV:48031 to 46271 | ORF Map | GBrowse
Note: this feature is encoded on the Crick strand.
Genetic position: -130 cM
Gene Ontology Annotations All HO GO evidence and references
  View Computational GO annotations for HO
Molecular Function
Manually curated
Biological Process
Manually curated
Cellular Component
Regulators 4 genes
Classical genetics
Large-scale survey
35 total interaction(s) for 24 unique genes/features.
Physical Interactions
  • Affinity Capture-MS: 4
  • Affinity Capture-RNA: 4
  • Affinity Capture-Western: 9
  • Co-crystal Structure: 1
  • Co-purification: 1
  • Reconstituted Complex: 3
  • Two-hybrid: 2

Genetic Interactions
  • Negative Genetic: 10
  • Positive Genetic: 1

Expression Summary
Length (a.a.) 586
Molecular Weight (Da) 66,089
Isoelectric Point (pI) 9.82
Phosphorylation PhosphoGRID | PhosphoPep Database
sequence information
ChrIV:48031 to 46271 | ORF Map | GBrowse
Note: this feature is encoded on the Crick strand.
Genetic position: -130 cM
Last Update Coordinates: 2011-02-03 | Sequence: 1996-07-31
Subfeature details
Most Recent Updates
Coordinates Sequence
CDS 1..1761 48031..46271 2011-02-03 1996-07-31
Retrieve sequences
Analyze Sequence
S288C only
S288C vs. other species
S288C vs. other strains
External Links All Associated Seq | Entrez Gene | Entrez RefSeq Protein | MIPS | Search all NCBI (Entrez) | UniProtKB
Primary SGDIDS000002386

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 (6,7). Ho initiates switching by recognizing and cleaving a degenerate 24 base-pair site at MAT making a double-stranded break (DSB) in DNA 8. Sequences at MAT are then replaced by copying new sequences from either HML or HMR 4. Although the Ho recognition sequence is present at both HML and HMR, the sequences are occluded by chromatin structure (9). Ho acts in a stoichiometric fashion but appears to act once and then become inactivated (10). Ho degradation by the ubiquitin-26s proteosome system is rapid with a half-life of 10 minutes (11).

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 (12, 13).

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 (14).

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 (14).

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) (15). Glycine 223 is essential for endonuclease activity and histidine 475 is important for full enzymatic activity (16, 15). In wild strains, the pattern of expression ensures that upon germination of a spore, the immobile yeasts are ultimately near qualified mating partners (17). 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 (13). The HO gene has been used extensively as a reagent to introduce DSBs in the study of recombination and for developing molecular reagents (18).

Last updated: 2005-12-13 Contact SGD

References cited on this page View Complete Literature Guide for HO
1) WINGE O and Roberts C  (1949) A gene for diploidization on yeast. C R Trav Lab Carlsberg Ser Physiol 24():341-346
2) Nasmyth K  (1993) Regulating the HO endonuclease in yeast. Curr Opin Genet Dev 3(2):286-94
3) Jensen R, et al.  (1983) Regulation of yeast mating-type interconversion: feedback control of HO gene expression by the mating-type locus. Proc Natl Acad Sci U S A 80(10):3035-9
4) Strathern JN, et al.  (1982) Homothallic switching of yeast mating type cassettes is initiated by a double-stranded cut in the MAT locus. Cell 31(1):183-92
5) Kostriken R, et al.  (1983) A site-specific endonuclease essential for mating-type switching in Saccharomyces cerevisiae. Cell 35(1):167-74
6) Nasmyth KA  (1982) Molecular genetics of yeast mating type. Annu Rev Genet 16:439-500
7) Herskowitz I  (1988) Life cycle of the budding yeast Saccharomyces cerevisiae. Microbiol Rev 52(4):536-53
8) Nickoloff JA, et al.  (1986) A 24-base-pair DNA sequence from the MAT locus stimulates intergenic recombination in yeast. Proc Natl Acad Sci U S A 83(20):7831-5
9) Haber JE  (1998) Mating-type gene switching in Saccharomyces cerevisiae. Annu Rev Genet 32:561-99
10) Jin Y, et al.  (1997) Ho endonuclease cleaves MAT DNA in vitro by an inefficient stoichiometric reaction mechanism. J Biol Chem 272(11):7352-9
11) Kaplun L, et al.  (2000) Functions of the DNA damage response pathway target Ho endonuclease of yeast for degradation via the ubiquitin-26S proteasome system. Proc Natl Acad Sci U S A 97(18):10077-82
12) Andrews BJ and Herskowitz I  (1989) Identification of a DNA binding factor involved in cell-cycle control of the yeast HO gene. Cell 57(1):21-9
13) Nasmyth K and Shore D  (1987) Transcriptional regulation in the yeast life cycle. Science 237(4819):1162-70
14) Cosma MP  (2004) Daughter-specific repression of Saccharomyces cerevisiae HO: Ash1 is the commander. EMBO Rep 5(10):953-7
15) Meiron H, et al.  (1995) Identification of the heterothallic mutation in HO-endonuclease of S. cerevisiae using HO/ho chimeric genes. Curr Genet 28(4):367-73
16) Ekino K, et al.  (1999) Functional analysis of HO gene in delayed homothallism in Saccharomyces cerevisiae wy2. Yeast 15(6):451-8
17) Taxis C, et al.  (2005) Spore number control and breeding in Saccharomyces cerevisiae: a key role for a self-organizing system. J Cell Biol 171(4):627-40
18) Haber JE  (1995) In vivo biochemistry: physical monitoring of recombination induced by site-specific endonucleases. Bioessays 17(7):609-20