Two different operons for the same function: comparison of the Salmonella typhimurium nrdAB and nrdEF genes.

By using a P22 phage-mediated cloning system, the nrdAB genes of Salmonella typhimurium (St), encoding a ribonucleotide reductase (RR) of class I, have been isolated. The coding regions of the St nrdAB operon show a very high identity with those of the homologous operon of Escherichia coli (Ec). Nevertheless, there are significant differences in their promoter regions since, although the promoters of both operons present two DnaA boxes, these boxes are located downstream from the transcription start point in St, being upstream in Ec. Moreover, the deduced amino-acid sequences of the St nrdAB showed a very limited overall identity (28%) with the products of St nrdEF, which encode a second class-I RR. Expression of St nrdAB and nrdEF is inducible by hydroxyurea, an inhibitor of RR activity. Alignment of the promoter regions of the nrdAB and nrdEF operons of both St and Ec reveals the presence of a consensus sequence. St is the first organism from which two different RR belonging to the same biochemical class are known.