Bacteriophage lambda cIII gene product has an additional function apart from inhibition of cII degradation

For lysogenization of Escherichia coli cells by bacteriophage lambda, funct ions of three lambda genes called c are necessary. The cI gene codes for a repressor that blocks activities of lytic promoters. However, early after i nfection, expression of cI is dependent on the function of the cII gene, co ding for a specific transcriptional activator. The cII protein is unstable in E. coli cells due to FtsH-mediated proteolysis. The cIII gene product is an inhibitor of the FtsH protease. Here we demonstrate that cIII may have another function apart from inhibition of cII degradation. We found that ov erexpression of the cII gene results in impaired lysogenization by phage la mbda, however simultaneous overexpression of the cIII gene abolished this n egative effect on lysogenization. Analysis of cII-mediated transcriptional activation of certain promoters at different levels of cII and cIII protein s in cells confirmed that observed effects cannot be explained assuming tha t the only role of cIII is inhibition of FtsH-mediated degradation of cII. We propose that cIII has an additional role apart from its well-known funct ion in indirect stabilization of cII. Apparently, cIII influences not only cII level but also activity of this transcriptional stimulator, especially at its high concentrations.