AN S13 CAPSID MUTANT THAT MAKES NO REPLICATIVE-FORM DNA

A temperature-sensitive mutant of phage S13 (t1069) has been isolated that fails to complement mutants in six genes tested. The reversion frequency of t1069 indicates that it is a single mutation rather than a sextuple one. The parental single-stranded DNA of t1069 is not converted to the double stranded replicative-form DNA (RF) at restrictive temperatures. Since the RF is needed for most if not all of the phage protein synthesis, the lack of complementation is explained. Heat stability tests have shown that t1069 has an altered capsid protein, and genetic crosses have located the mutant in gene IIIb. Thus at restrictive temperatures the altered IIIb capsid protein of t1069 blocks the initial steps of the normal infectious process that either precede or coincide with the replication of parental single-stranded DNA to RF. © 1969.