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The Escherichia coli ribonucleotide reductase genes (nrd genes) were mutagenized at random. Point mutations were introduced in vitro into a recombinant nrd plasmid. Transformants were initially screened for altered tolerance toward the drug hydroxyurea and further characterized by enzymatic and immunological methods. The screening procedure could pick out defects in either of the two subunits of ribonucleotide reductase. Cells carrying the nrd plasmid pPS2 were earlier shown to have levels of ribonucleotide reductase molecules that were 10 to 20 times higher than those in wild-type cells. We now demonstrate that the enzymatic activity in gently lysed pPS2-containing cells on cellophane disks is six times higher than in wild-type cells. Supplementation of the pPS2-containing lysates with a purified thioredoxin system results in a further 4.5-fold stimulation of the enzymatic activity, which implies a functional shortage of the electron donor system(s) for ribonucleotide reduction in pPS2-containing cells.

Mutationally altered ribonucleotide reductase from Escherichia coli: characterization of mutations isolated on multicopy plasmids