Question

How do the \(\lambda\) repressor and the cro protein affect the transcription from \(P_{\mathrm{R}}\) and \(P_{\mathrm{RM}}\) ? Explain where these proteins are binding to cause their effects.

Short answer

The λ repressor and cro proteins bind to operator sites \(O_{\mathrm{R}}\) on the phage DNA and act by blocking certain promoters. The λ repressor inhibits the \(P_{\mathrm{R}}\) promoter and promotes \(P_{\mathrm{RM}}\) promoter, promoting lysogeny. On the other hand, cro protein, after reaching certain levels, can inhibit both \(P_{\mathrm{R}}\) and \(P_{\mathrm{RM}}\) promoters, promoting the lytic cycle.

Step-by-step solution

Understanding the λ Repressor Protein

The λ repressor protein (cI) is produced in a λ bacteriophage when it wants to enter lysogeny. The cI protein binds to the operator sites \(O_{\mathrm{R}}\) and \(O_{\mathrm{L}}\) on the phage DNA. By binding to \(O_{\mathrm{R2}}\) and \(O_{\mathrm{R3}}\), the cI protein represses the \(P_{\mathrm{R}}\) promoter and inhibits transcription of cro gene. The binding of cI to \(O_{\mathrm{R1}}\) further promotes its own expression by activating the \(P_{\mathrm{RM}}\) promoter.

Understanding the Cro Protein

The cro protein, on the other hand, is produced in the early phase of the lytic cycle in λ phage. This protein also binds to \(O_{\mathrm{R}}\) sites, but its binding affinities for \(O_{\mathrm{R1}}\), \(O_{\mathrm{R2}}\) and \(O_{\mathrm{R3}}\) are different from cI protein. The Cro protein prefers to bind to \(O_{\mathrm{R3}}\) first, then \(O_{\mathrm{R2}}\), and lastly to \(O_{\mathrm{R1}}\). So, when Cro binds to \(O_{\mathrm{R3}}\), it represses \(P_{\mathrm{RM}}\) and prevents the synthesis of cI protein. When the Cro protein levels are high enough to bind to \(O_{\mathrm{R2}}\), it also starts to repress \(P_{\mathrm{R}}\), ensuring its own synthesis is halted.

Conclusion of Protein Regulation Impact

In conclusion, both the λ repressor and cro proteins bind to the same operator sites but have different effects due to their different binding preferences. The λ repressor encourages lysogeny by inhibiting the \(P_{\mathrm{R}}\) promoter and promoting \(P_{\mathrm{RM}}\) promoter, whereas the cro protein promotes the lytic cycle by inhibiting both promoters when its levels are sufficiently high.