Question

Cro and cI proteins both have helix-turn-helix DNA binding domains. What effect would replacing cro’s DNA binding domain with that of cI’s have on the life-cycle of lambda phage? Explain briefly

Answer 1

CI is a transcription factor in lambda phage which is also known as the lambda repressor. This repressor is required for lysogenic cycle of the phage. Each repressor has two cI proteins forming a homodimer via C-terminal domains. When two homodimers of this repressor bound via their C-terminal domain (homodimer) bind to adjacent sites on DNA, cro gene is inhibited. Further, this co-operative binding activates the cI gene. Cro gene encodes for Cro protein while cI gene encodes for cI protein.

The lysogenic cycle is maintained if there is cI protein present. However, if cI proteins are absent, Cro will mediate its action, causing shift to lytic cycle. In absence of cI, there is binding of RNA polymerase to promoter of Cro, leading to cro transcription. When cI is present, it binds to OR1 and OR2, allowing RNA polymerase to bind pRM and induce transcription of cI. Cro can bind to OR3 and thus, prevents the access of RNA polymerase to PRM promoter. Hence, this inhibits transcription of cI.

If the Cro DNA binding region is replaced by the DNA binding region of cI, then the phage will undergo lysogenic cycle only. Cro is never able to bind to OR3 and inhibit the PRM promoter. Instead Cro will act as cI and bind to OR1 and OR2. Hence, OR3 is free and thus, transcription will start from PRM. This will lead to transcription of cI, leading to lysogenic cycle. Cro transcription is inhibited in this case. Hence, cI will always be active, maintaining the phage in lysogenic cycle.