Gene-III protein of filamentous phages: evidence for a carboxyl-terminal domain with a role in morphogenesis.

A filamentous phage derivative, fCA55, bearing a nonpolar deletion in gene III, has been constructed and characterized to study the functions of that gene. The deletion eliminates most of gene III without disturbing its reading frame or the putative promoter for the downstream gene, VI. Therefore it is assumed that any abnormalities exhibited by fCA55 are a direct effect of the gene-III lesion itself, and not polar effects on other genes. fCA55 Is abnormal in two respects. First, it is noninfective; in this it resembles another nonpolar gene-III deletion mutant, fKN16, which is missing 507 bp encompassing roughly the first half of the gene. Second, it is secreted as polyphage--very long particles containing many unit-length DNA molecules; in this respect, fCA55 differs from fKN16. When the viral proteins of these two mutants were analyzed with antibody directed against gene-III protein, it was found that fKN16 contains an altered gene-III protein, while fCA55 is unreactive. It was concluded that the gene-III protein has two functional domains: the N-terminal domain, missing in both mutants, is required for viral infectivity; while the C-terminal domain, partly missing in fCA55 but retained in fKN16, is incorporated into the virion, and is responsible for the protein's role in generating normal, unit-length particles.