Control of bacteriophage lambda repressor synthesis after phage infection: The role of the N, cII, cIII and cro products

Abstract

During the infectious cycle of phage λ, the products of three phage genes ( N, cII, and cIII) promote λ repressor synthesis. The N protein does not directly increase the repressor level. Instead, it increases the synthesis of the cII and cIII proteins, which in turn, activate transcription from the promoter p re of cI (the repressor structural gene). At low phage multiplicities, the amount of cI expression is limited by the number of cI and cIII, but not N or cII templates. Phage DNA replication also promotes cI expression at low phage multiplicities. Since cI expression does not require phage DNA replication at higher multiplicities, the major role of replication is probably to increase the number of limiting gene templates. cro product turns off repressor synthesis by reducing synthesis of the N, cIII, and cII proteins. All three activator proteins are unstable, so their levels decline to suboptimal values midway in the infective cycle, resulting in reduced cI expression and λ repressor synthesis, cro product directly reduces gene expression in the two operons containing cIII and cII. cro product further reduces cIII and cII expression by lowering N protein synthesis, cro product appears to be an indirect regulator of repressor synthesis from p re, effective only because the cII and cIII proteins are unstable, cro product also behaves as a much less efficient represser than the cI protein, possibly because it reduces its own synthesis and is unstable.