Q

a-specific URS

No transcription of a-specific genes a-specific URS

a2 dimer asg

Transcription of a-specific genes a-specific URS

Transcription of a-specific genes a2 dimer a2

No transcription of a-specific genes

^ FIGURE 22-12 Activity of MCM1 in a and a yeast cells. MCM1 binds as a dimer to the P site in a-specific and a-specific upstream regulatory sequences (URSs), which control transcription of a-specific genes and a-specific genes, respectively. (a) In a cells, MCM1 stimulates transcription of a-specific genes. MCM1 does not bind efficiently to the P site in a-specific URSs in the absence of a1 protein. (b) In a cells, the activity of MCM1 is modified by its association with a1 or a2. The a1-MCM1 complex stimulates transcription of a-specific genes, whereas the a2-MCM1 complex blocks transcription of a-specific genes. The a2-MCM1 complex also is produced in diploid cells, where it has the same blocking effect on transcription of a-specific genes (see Figure 22-11c).

a factor a-factor' receptor a factor a-factor receptor

Pheromone signaling ■

Cell-cycle arrest Morphogenesis

Cell-cycle arrest Morphogenesis

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