Satellite bacteriophage P4: Characterization of mutants in two essential genes

Abstract

P4 is a helper-dependent bacteriophage which can use the late gene products of temperate phage P2 to encapsulate P4 DNA ( Six, 1963; Six and Lindqvist, 1970; Gibbs, 1972; Six and Klug, 1973). P4 phage heads (d = 450 Å) contain only one-third the volume of P2 phage heads (d = 620 Å; Inman et al., 1971) , and heads of the P4 size are not detected during a normal P2 infection (D. Walker, personal communication). Thus P4 directs the formation of 450 Å phage heads. P4 causes P2 prophage late genes to be expressed in the presence of immunity ( Six and Klug, 1973), and without excision or replication of the prophage genome ( Six and Lindqvist, 1971). In the absence of a helper, P4 DNA can replicate ( Lindqvist and Six, 1971). We have isolated and characterized mutants in two essential P4 genes. P4 gene A mutants are unable to synthesize P4 DNA, but they retain the ability to transactivate 2 2 Thomas (1970) used the term transactivation to describe the induction of gene expression from a λ prophage by a superinfecting heteroimmune phage. P2 prophage genes under nonpermissive conditions. Thus the A gene product may participate directly in the process of P4 DNA replication. P2-lysogenic, nonpermissive cells infected with P4 gene A mutants synthesize empty phage heads, 80% of which are intermediate in size between P4 heads and P2 heads. Heads of this intermediate size are also formed in small quantity during a normal P2 infection. The inability of P4 gene A mutants to synthesize P4-size heads may be due to a lack of replicating DNA or to lack of a size-directing protein. P4 gene B is defined by one temperature-sensitive mutation which is partially dominant to P4 wild type at 42 °. At the nonpermissive temperature this mutant can synthesize P4 DNA and cause a P2 prophage to be transcribed, but cannot cause the formation of head-like particles. Unlike P4 wild type, this mutant kills nonlysogenic cells at 42 °, and greatly depresses the synthesis of DNA, RNA, and protein.