2 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 Salmo nella typhimurium (St), encoding a ribonucleotide reductase (RR) of cl ass I, have been isolated. The coding regions of the St nrdAB operon s how a very high identity with those of the homologous operon of Escher ichia coli (Ec). Nevertheless, there are significant differences in th eir promoter regions since, although the promoters of both operons pre sent two DnaA boxes, these boxes are located downstream from the trans cription start point in St, being upstream in Ec. Moreover, the deduce d amino-acid sequences of the St nrdAB showed a very limited overall i dentity (28%) with the products of St nrdEF, which encode a second cla ss-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 c onsensus sequence. St is the first organism from which two different R R belonging to the same biochemical class are known.