Specific inhibition of host processes by bacteriophage N4 gene products

Coliphage N4 directs specific and rapid inhibition of host DNA replication immediately after infection, while host transcription and translation continue throughout N4 infection. Two early-expressed phage N4 gene products responsible for host inhibition have now been identified and their mechanisms of action characterized: N4 gp6, a basic 52-residue peptide, and N4 gp8, a basic 71-residue peptide.;N4 gp6 is an inhibitor of E. coli cell division with a bacteriostatic effect on cell growth. Gp6 acts after nucleoid separation and before the first observable step in cell division, formation of the FtsZ ring at midcell. Gp6 does not act through the known SOS-response pathway of cell division inhibition. While gp6 is nonessential for N4 phage, it contributes to phage yield. The specific target of gp6 remains to be determined.;N4 gp8 specifically inhibits E. coli DNA replication elongation with a bacteriostatic effect. Gp8 is necessary for inhibition of host DNA replication during N4 infections. Although gp8 is nonessential for N4 phage, gp8's function to inhibit host DNA replication increases phage yield.;Gp8 colocalizes with and crosslinks to E. coli DNA polymerase III, the replicative polymerase, in vivo. Gp8 has been expressed and purified from E. coli cells, and the purified protein inhibits processive strand elongation by DNA polymerase III in vitro. Gp8 interferes with polymerase processivity by inhibiting DNA polymerase clamp loader subcomplex-catalyzed loading of DNA polymerase processivity clamp subunit onto DNA templates.;Gp8 targets the HolA (delta) subunit of DNA polymerase III, located in the clamp loader complex. Mutations conferring resistance to gp8 have been selected in the gene encoding HolA and identified in the first and third domains of HolA. Furthermore, gp8 has been found to interact with HolA in vitro.;Thus, N4 encodes specific inhibitors of E. coli cell division and DNA replication. Gp8's interference with HolA activity in the replicative polymerase represents a novel mechanism of bacterial growth inhibition.