Escherichia coli DNA polymerase I. Sequence characterization and secondary structure prediction.

The primary sequence of DNA polymerase I from Escherichia coli K12 as derived from the DNA sequence (Joyce, C. M., Kelley, W. S., and Grindley, N. D. F. (1982) J. Biol. Chem. 257, 1958-1964) has been verified. Protein sequencing through eight cycles of the Klenow large fragment yields a unique sequence corresponding to residues 324 to 331 from the translated DNA sequence and defines the subtilisin cleavage site for formation of the large and small fragments as Thr323-Val324. Site-specific cleavage of whole enzyme and large fragment at cysteines and sizing of the resulting fragments verify the location of the two cysteines at residues 262 and 907 as assigned by the DNA sequencing. Isolation of tryptic peptides derived from DNA polymerase I yielded unique peptides whose composition exactly corresponded to theoretical tryptic peptides derived from the translated DNA sequence. Identification of the expected carboxyl-terminal tryptic peptide and carboxypeptidase digestion of whole enzyme and large fragment confirm histidine-928 as the carboxylterminus. A secondary structure prediction is made using the available primary sequence data. The model contains 43% alpha helix, 17% beta-structure, 58 beta-turns, and several interesting super-secondary structure elements.