Abnormal Forms of Filamentous Phage Virions and Phage DNA

Abstract

A careful analysis of mistakes that occur in biological systems often leads to useful insights into the normal process. Whereas the icosahedral single-stranded DNA phages are constrained to package a fixed “headful” of DNA, the filamentous phages exhibit great flexibility in the amount of DNA they accept. One of the earliest papers about these phages (Marvin and Hoffmann-Berling 1963) contained the suggestion that phage of different lengths exist in wild-type stocks. During a study of complementation between different conditionally lethal mutants, two groups (Salivar et al. 1967; Scott and Zinder 1967) discovered independently that the 1–10% turbid plaques formed on nonpermissive hosts arose from “heterozygous” phage; these plaques contained both parental phage types, which maintained themselves by a process of continuous complementation. Electron microscopy and antiserum inactivation experiments suggested that these plaques arose from double-length phage (Scott and Zinder 1967; Salivar et al. 1967; Beaudoin and Pratt 1974). A much lower frequency of trimer phage was also noted. Phage produced after infection of nonpermissive strains with amber mutants of genes III and VI alone consisted of up to 70% dimer phage as well as many trimer and even tetramer phage (Pratt et al. 1969). The products of genes III and VI are implicated in phage assembly; the gene-III protein is found at the end of the phage and is necessary for adsorption to the host cell (Pratt et al. 1969).

Early attempts to find double-length, circular phage DNA in the dimers were unsuccessful (Salivar et al. 1967). After examination...