Mutants of the bacteriophage f2: VIII. Control mechanisms for phage-specific syntheses

Abstract

Amber mutations in the coat protein cistron of the RNA phage f2 alter the normal process of control of synthesis both of viral RNA and of viral RNA polymerase. These effects depend on the position of the amber mutation and on the suppressor properties of the host bacterium. When infecting Su− bacteria, which lack a suppressor gene, mutants (sus-3) carrying an amber codon near the N-terminus of the virus coat protein cistron are pleiotropic. For initially little single- or double-stranded viral RNA or viral RNA polymerase is made, although the parental phage RNA is converted into a double-stranded form. Infection of bacterial hosts such as Su-II, which partially suppress the amber mutation, with the sus-3 mutant results in normal synthesis of single-stranded and 14 to 18 s double-stranded viral RNA. As in f2 wild-type infected cells, all enzymes needed for this synthesis are made very early in infection. However, late in infection, all available single-stranded viral RNA (including RNA from superinfecting wild-type phage) is converted to small (7 s) double-stranded RNA; this transformation is not merely the consequence of lack of viral coat protein to encapsulate the RNA, but is dependent on protein synthesis late in infection. These results provide support for the existence of a phage-induced enzyme (enzyme I) which converts single-stranded viral RNA into a double-stranded form. An amber mutant (sus-11) carrying a mutation near the middle of the virus coat protein cistron, makes, on both non-permissive hosts (Su-II and Su−), normal amounts of single-stranded RNA, but excess amounts of viral polymerase (enzyme I) and 7 s double-stranded RNA. These results are interpreted in terms of a model coupling genetic translation and transcription. Viral RNA is translated in two distinct states: as free (parental) RNA, which makes RNA polymerase(s); and as nascent messenger still bound to its double-stranded RNA template, which makes coat and other phage proteins synthesized late in infection. The translation into protein of the N-terminal region of the coat cistron is necessary for continued synthesis of the nascent messenger RNA. When RNA is synthesized, under conditions where insufficient coat protein is made, as in mutant-infected cells, single-stranded progeny RNA molecules appear analogous to parental RNA molecules and synthesize enzyme I.