Protein B1 of ribonucleotide reductase. Direct analytical data and comparisons with data indirectly deduced from the nucleotide sequence of the Escherichia coli nrdA gene.

The total composition, the N-terminal amino acid sequence, and the amino acid sequences of four internal regions have been determined for the ribonucleotide reductase large subunit, protein B1, prepared from a recombinant lambda-lysogenic Escherichia coli K strain, which overproduces the enzyme 30-50-fold. The data have been compared with those previously reported for B1 prepared from a thymine-starved E. coli B strain and with the indirectly derived primary structure of B1 recently reported from the nucleotide sequence of the E. coli K nrdA gene. Two major differences to these results were found. First, the B1 polypeptides started with initiator Met-1 (45%), Asn-2 (30%) or Gln-3 (15%), demonstrating a different type of N-terminal heterogeneity than that found earlier. Secondly, the total amino acid composition as derived from hydrolyzed protein B1 differed substantially from the amino acid composition derived from the nucleotide data. This has the consequence that Cys, Arg, Thr and possibly Val and Ser appear more frequently whereas Asx, Glx, Tyr and possibly Gly appear less frequently in the nucleotide-derived data as compared to direct protein hydrolysates. We suggest usage of other reading frames in the approximate area of residues 630-700 of the primary structure of the nrdA gene to compensate for these discrepancies and for the relatively high incidence of uncommon codons in the reading frame proposed for this area of the gene. Such changes have implications on the previously assigned putative active-site region of protein B1.