CLONING AND SEQUENCING OF THE GENES FROM SALMONELLA-TYPHIMURIUM ENCODING A NEW BACTERIAL RIBONUCLEOTIDE REDUCTASE

A plasmid library of Salmonella typhimurium was used to complement a t emperature-sensitive nrdA mutant of Escherichia coli. Complementation was obtained with two different classes of plasmids, one carrying the E. coli nrdAB-like genes and the second containing an operon encoding a new bacterial ribonucleotide reductase. Plasmids harboring these new reductase genes also enable obligately anaerobic nrdB::Mud1 E. coli m utants to grow in the presence of oxygen. This operon consists of two open reading frames, which have been designated nrdE (2,145 bp) and nr dF (969 bp). The deduced amino acid sequences of the nrdE and nrdF pro ducts include the catalytically important residues conserved in ribonu cleotide reductase enzymes of class I and show 25 and 28% overall iden tity with the R1 and R2 proteins, respectively, of the aerobic ribonuc leoside diphosphate reductase of E. coli. The 3' end of the sequenced 4.9-kb fragment corresponds to the upstream region of the previously p ublished proU operon of both S. typhimurium and E. coli, indicating th at the nrdEF genes are at 57 min on the chromosomal maps of these two bacterial species. Analysis of the nrdEF and proU sequences demonstrat es that transcription of the nrdEF genes is in the clockwise direction on the S. typhimurium and E. coli maps.