Revisiting the lysogenization control of bacteriophage lambda. Identification and characterization of a new host component, HflD.

Upon infection to the Escherichia coli cell, the genome of bacteriophage lambda either replicates to form new progenies (lytic growth) or integrates into the host chromosome (lysogenization). The lambda CII protein is a key determinant in the lysis-lysogeny decision. It is a short-lived transcription activator for the lambda genes essential for lysogeny establishment. In this study, we isolated a new class of hfl (high frequency lysogenization) mutants of E. coli, using a new selection for enhancement of CII-stimulated transcription. The gene affected was termed hflD, which encodes a protein of 213 amino acids. An hflD-disrupted mutant indeed showed an Hfl phenotype, indicating that HflD acts to down-regulate lysogenization. HflD is associated peripherally with the cytoplasmic membrane. Its interaction with CII was demonstrated in vitro using purified proteins as well as in vivo using the bacterial two-hybrid system. Pulse-chase examinations demonstrated that the HflD function is required for the rapid in vivo degradation of CII, although it interfered with FtsH-mediated CII proteolysis in an in vitro reaction system using detergent-solubilized components. We suggest that HflD is a factor that sequesters CII from the target promoters and recruits it to the membrane where the FtsH protease is localized.