MUTANTS AT CONSERVED POSITIONS IN GENE-IV, A GENE REQUIRED FOR ASSEMBLY AND SECRETION OF FILAMENTOUS PHAGES

The filamentous phage protein pIV is required for assembly and secreti on of the virus and possesses regions homologous to those found in a n umber of Gram-negative bacterial proteins that are essential component s of a widely distributed extracellular protein-export system. These p roteins form multimers that may constitute an outer membrane channel t hat allows phage/protein egress. Three sets of f1 gene IV mutants were isolated at positions that are absolutely (G355 and P375) or largely (F381) conserved amongst the 16 currently known family members. The G3 55 mutants were non-functional, interfered with assembly of pIV(+) pha ge, and made Escherichia coli highly sensitive to deoxycholate. The P3 75 mutants were non-functional and defective in multimerization. Many of the F381 mutants retained substantial function, and even those in w hich charged residues had been introduced supported some phage assembl y. Some inferences about the roles of these conserved amino acids are made from the mutant phenotypes.