Selection, identification, and genetic analysis of random mutants in the cloned primase/helicase gene of bacteriophage T7.

T7 gene 4 specifies two overlapping proteins 4A, a 566-amino acid primase/helicase, and 4B, a 503-amino acid helicase whose initiation codon is the 64th codon of the 4A protein. The 4A' gene, which has a leucine codon replacing the 4B initiation codon, specifies a single 566-amino acid protein that can provide the primase and helicase functions required for normal T7 growth. We selected N-methyl-N'-nitro-N-nitrosoguanidine mutants in the cloned 4A' gene that no longer support the growth of a phage that completely lacks gene 4. Genetic mapping of the 76 mutations found them to be distributed throughout the protein, including both the N-terminal and C-terminal halves of the molecule thought to represent primase and helicase domains, respectively. Complementation tests with partially and completely defective phage showed that all but five of the mutants lacked helicase function but retained primase function. The other five, which lacked both functions, all made short proteins, including one missing only 60 amino acids. No mutations lacked only primase function, and none mapped within the first 105 amino acids, which includes the 63-amino acid region unique to 4A that contains elements required to recognize primase sites. Forty-six mutations were sequenced and included 27 missense mutations affecting 25 amino acids. Many mutations in the N-terminal half of the protein affected its solubility in cell extracts. Mutations in the C-terminal half clustered in or near five helicase consensus sequences. Biochemical analysis of nine of the mutant proteins is described in the accompanying paper (Washington, M. T., Rosenberg, A. H., Griffin, K., Studier, F. W., and Patel, S. S. (1996) J. Biol. Chem. 271, 26825-26834).