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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