Identification by immunobinding assay of the polypeptide coded by the DNA polymerase gene of bacteriophage T5 and its amber mutants and the direction of transcription of the gene.

We identified by immunobinding assay the polypeptides synthesized as the result of amber mutations in the DNA polymerase gene of bacteriophage T5. Comparison of the size of such polypeptides revealed the order of mutagenic loci of these mutations and the direction of transcription of the gene. Extracts of cells infected with wild-type T5 and with five amber mutants of the polymerase gene (D7, D8, D9, am1, and am6) were prepared, and the proteins were resolved by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. After transfer of the proteins to a nitrocellulose sheet, a radioimmunolabeling technique was used to identify the T5 DNA polymerase and its amber mutant polypeptides. Based on the relative sizes of the polypeptides, the transcription of the T5 DNA polymerase gene was determined to proceed in the order D7, D8, am1, D9, and am6. The molecular weights of the DNA polymerase polypeptides coded by D8, am1, D9, am6, and T5+ were 23,000, 45,000, 75,000, 83,000, and 96,000, respectively. The D7-coded polypeptide was not detectable. These data suggest that the carboxyl-terminal region of the enzyme is essential for the polymerase function.