Recombinant Enterobacteria phage lambda Regulatory protein cro (cro)

Plays an essential role in the switch from lysogenic to lytic state and in fine-tuning the lytic phase activation. Mediates the de-repression of the lytic promoters from repression by cI at an early stage of prophage induction. Turns down the expression of the transcriptional activator cII, a factor that initiates the expression of repressor cI, the major component promoting latency. Additionally, cro is able to directly bind to the promoter region of cI thereby preventing the transcription of cI repressor. These two mechanisms ultimately lead to a strong decrease in the amount of cI that will become unable to maintain the latent phase, allowing the transition to lytic growth. Low level of Cro also stimulates the lytic promoters while cI repressor is still present. Cro-mediated up-regulation of pR and pL ultimately leads to its own increased expression and to a successful switch to lytic growth. The second function assigned to Cro is the fine-tuning of the lytic phase activation. Indeed, cro initially diminishes the strength of activation by turning down the expression of lytic genes as well as its own expression.