Identification of the stable free radical tyrosine residue in ribonucleotide reductase.

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Abstract
The small subunit of iron‐dependent ribonucleotide reductases contains a stable organic free radical, which is essential for enzyme activity and which is localized to a tyrosine residue. Tyrosine‐122 in the B2 subunit of Escherichia coli ribonucleotide reductase has been changed into a phenylalanine. The mutation was introduced with oligonucleotide‐directed mutagenesis in an M13 recombinant and verified by DNA sequencing. Purified native and mutant B2 protein were found to have the same size, iron content and iron‐related absorption spectrum. The sole difference observed is that the mutant protein lacks tyrosyl radical and enzymatic activity. These results identify Tyr122 of E. coli protein B2 as the tyrosyl radical residue. An expression vector was constructed for manipulation and expression of ribonucleotide reductase subunits. It contains the entire nrd operon with its own promoter in a 2.3‐kb fragment from pBR322. Both the B1 and the B2 subunits were expressed at a 25‐35 times higher level as compared to the host strain.