Deoxyribonucleotide synthesis in an Escherichia coli mutant (H 1491) which lacks ribonucleotide reductase subunit B2

An iron-sensitive mutant of E. coli with a Mudl phage insertion in the nrdB gene lacks subunit B2 of the key enzyme of DNA synthesis, ribonucleotide reductase. Nevertheless, these cells are capable of growing in minimal media under anaerobic conditions, indicating a second enzyme or pathway for deoxyribonucleotide synthesis. We here show that ribonucleotide reduction cannot be unambiguously measured in bacterial extracts whereas phosphorylase-catalyzed deoxyribosyl transfer does occur; however these salvage reactions could not function in vivo in the absence of deoxyribosides. It is suggested that the cells possess a specific, anaerobic ribonucleotide reductase which escapes detection under aerobic standard conditions, similar to the situation found in strictly anaerobic methanogens.