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Production of deoxyribonucleotides for DNA synthesis is an essential and tightly regulated process. The class Ia ribonucleotide reductase (RNR), the product of thenrdAB genes, is required for aerobic growth ofEscherichia coli . In catalyzing the reduction of ribonucleotides, two of the cysteines of RNR become oxidized, forming a disulfide bond. To regenerate active RNR, the cell uses thioredoxins and glutaredoxins to reduce the disulfide bond. Strains that lack thioredoxins 1 and 2 and glutaredoxin 1 do not grow because RNR remains in its oxidized, inactive form. However, suppressor mutations that lead to RNR overproduction allow glutaredoxin 3 to reduce sufficient RNR for growth of these mutant strains. We previously described suppressor mutations in thednaA anddnaN genes that had such effects. Here we report the isolation of new mutations that lead to increased levels of RNR. These include mutations that were not known to influence production of RNR previously, such as a mutation in thehda gene and insertions in thenrdAB promoter region of insertion elements IS1 and IS5 . Bioinformatic analysis raises the possibility that IS element insertion in this region represents an adaptive mechanism innrdAB regulation inE. coli and closely related species. We also characterize mutations altering different amino acids in DnaA and DnaN from those isolated before.

Mutations at Several Loci Cause Increased Expression of Ribonucleotide Reductase in Escherichia coli