Chapter: Genetics and Molecular Biology: Lambda Phage Genes and Regulatory Circuitry
Role of Cro Protein
The Role of Cro Protein
Once lambda has started down the lytic
pathway of development, it should not waver in its efforts to maximize
efficiency. The three regu-latory genes we discussed earlier, CI, CII,
and CIII all act to turn off genes
used in the lytic cycle. Their presence would be deleterious to lytic growth.
Not surprisingly, then, the phage makes a protein to turn down excessive
synthesis of these three proteins. It is called the Cro protein, for control of
repressor synthesis. First, Cro binds to an operator adjacent to the promoter
for the maintenance synthesis of CI repressor in lysogens, pRM (Fig. 14.6). Thus, no CI is made via this route.
Later, when Cro reaches higher concentrations, it binds to the operators
adjacent to pL and pR and represses
transcription from them as well. This secondary effect reduces the synthesis of
the phage CII and CIII proteins.
Figure 14.6 Cro protein first inactivatespRMand at higher concentrationsreduces activity of pL and pR.
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