Gue

asg

MATal =

o^xaD

MATal =

o^xaD

Eh asg hsg

▲ FIGURE 22-11 Transcriptional control of cell type-specific genes in S. cerevisiae. The coding sequences carried at the MAT locus differ in haploid a and a cells and in diploid cells. Three type-specific transcription factors (a1, a2, and a1) encoded at the MAT locus act with MCM1, a constitutive transcription factor produced by all three cell types, to produce a distinctive pattern of gene expression in each of the three cell types. asg = a-specific genes/mRNAs; asg = a-specific genes/mRNAs; hsg = haploid-specific genes/mRNAs.

MCM1 and a1-MCM1 Complexes Activate Gene Transcription

In a cells, homodimeric MCM1 binds to the so-called P box sequence in the upstream regulatory sequences (URSs) of a-specific genes, stimulating their transcription (Figure 22-12a). Transcription of a-specific genes is controlled by two adjacent sequences—the P box and the Q box—located in the URSs associated with these genes. Although MCM1 alone binds to the P box in a-specific URSs, it does not bind to the P box in a-specific URSs. Thus a cells do not transcribe a-specific genes.

In a cells, which produce the a1 transcription factor encoded by MATa, the simultaneous binding of MCM1 and a1 to PQ sites occurs with high affinity (Figure 22-12b). This binding turns on transcription of a-specific genes. Therefore a-specific transcription is a simple matter of a single transcription factor binding to its target genes, while a-specific

MCM1 dimer

MCM1 dimer a-specific URS

No transcription of a-specific genes a-specific URS

Transcription of a-specific genes a-specific URS

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