Abstract
Twenty-seven new mutations in the structural gene for the Salmonella typhimurium bacteriophage P22 tailspike protein have been isolated, mapped using a powerful plasmid-based genetic system and their DNA sequence changes determined. The mutations were generated by hydroxylamine treatment of the cloned gene on a plasmid expression vector. Assaying the activity of the tailspike protein produced from this plasmid and screening for plasmid mutants were accomplished by the in situ complementation of P22 capsids imbedded in soft agar to produce infectious phage. Deletion mutations in the cloned gene have been constructed by a two step procedure involving oligonucleotide linker insertion and in vitro deletion by restriction endonuclease digestion. The deletions, whose physical endpoints were determined by DNA sequencing, define 12 genetic and physical intervals into which the new mutations were mapped by marker rescue experiments. These deletions were transferred to phage P22 by recombination and used to map mutations carried on plasmids. Following mapping, the nucleotide change for each of the mutations was determined by DNA sequencing. The majority were absolute missense mutations although both amber and ochre nonsense mutations were also identified in the protein coding portion of the gene. The suppression pattern of the nonsense mutations was determined on several nonsense suppressors. Four of the mutations cause severely depressed levels of tailspike protein expression from both the cloned gene on the plasmid expression vector and from P22 phage carrying these mutations. These mutations were identified as nucleotide changes in what is probably the P22 late operon transcription terminator which immediately follows the tailspike protein coding sequence.