6. Regulation of HO expression

17.6 Regulation of HO expression

Production of the HO endonuclease is regulated at the level of gene transcription. There are three separate control systems:

• HO is under mating-type control. It is not synthesized in MATa/MATα diploids. In teleological terms, we may think that there is no need for switching when both MAT alleles are expressed anyway.
  
• HO is transcribed in mother cells but not in daughter cells.
  
• HO transcription also responds to the cell cycle. The gene is expressed only at the end of the G1 phase of a mother cell.

Figure 17.11 Switching occurs only in mother cells; both daughter cells have the new mating type. A daughter cell must pass through an entire cycle before it becomes a mother cell that is able to switch again.

The timing of nuclease production explains the relationship between switching and cell lineage. Figure 17.11 shows that switching is detected only in the products of a division; both daughter cells have the same mating type, switched from that of the parent. The reason is that the restriction of HO expression to G1 phase ensures that the mating type is switched before the MAT locus is replicated, with the result that both progeny have the new mating type.

Figure 17.12 Three regulator systems act on transcription of the HO gene. Transcription occurs only when all repression is lifted.

Cis-acting sites that control HO transcription reside in the 1500 bp upstream of the gene. The general pattern of control is that repression at any one of many sites, responding to several regulatory circuits, may prevent transcription of HO. Figure 17.12 summarizes the types of sites that are involved.

Mating type control resembles that of other haploid-specific genes. Transcription is prevented (in diploids) by the a1/α2 repressor. There are 10 binding sites for the repressor in the upstream region. These sites vary in their conformity to the consensus sequence; we do not know which and how many of them are required for haploid-specific repression.

The control of HO transcription involves interplay between a series of activating and repressing events. The genes SWI1 V5 are required for HO transcription. They function by preventing products of the genes SIN1 V6< from repressing HO. The SWI genes were discovered first, as mutants unable to switch; then the SIN genes were discovered for their ability to release the blocks caused by particular SWI mutations. SWI-SIN interactions are involved in both cell-cycle control and the restriction of expression to mother cells.

Some of the SWI and SIN genes are not specifically concerned with mating type, but are global regulators of transcription, whose functions are needed for expression of many loci. They include the activator complex SWI1,2,3 and the loci SIN1 V4 that code for chromosomal proteins. Their role in mating type expression is incidental. The "real" regulator is therefore the SWI protein that counteracts the general repression system specifically at the HO locus.

Cell-cycle control is conferred by 9 copies of an octanucleotide sequence called URS2. A copy of the consensus sequence can confer cell-cycle control on a gene to which it is attached. A gene linked to this sequence is repressed except during a transient period toward the end of G1 phase. SWI4 and SWI6 are the activators that release repression at URS2. Their activity depends on the function of the cell-cycle regulator CDC28, which executes the decision that commits the cell to divide (see 27 Cell cycle and growth regulation).

The target for restricting expression to alternate generations is the activator SWI5 (which antagonizes a general repression system exercised by SIN3,4). In mutants that lack these functions, HO is transcribed equally well in mother and daughter cells. This system acts on URS1 elements in the far upstream region.

SWI5 is not itself the regulator of mother-cell specificity, but is antagonized by Ash1p, a repressor that accumulates preferentially in daughter cells at the end of anaphase. Mutations in ASH1 allow daughter cells to switch mating type. The basis for the accumulation of Ash1p is not known. The possibility that it involves transport of the protein or mRNA is suggested by the identification of mutations in an actomyosin-dependent system that block its accumulation. Its presence prevents SWI5 from activating the HO gene.It might work either by binding to the URS1 promoter sequences or by binding to SWI5 protein. When the daughter cell grows to become a mother cell, the concentration of Ash1p is diluted, and it becomes possible to express the HO gene again (Bobola et al., 1996).

Research

Bobola, N. et al. (1996). Asymmetric accumulation of Ash1p in postanaphase nuclei depends on a myosin and restricts yeast mating-type switching to mother cells. Cell 84, 699-709.