A 2ND CLASS-I RIBONUCLEOTIDE REDUCTASE IN ENTEROBACTERIACEAE - CHARACTERIZATION OF THE SALMONELLA-TYPHIMURIUM ENZYME

The nrdA and nrdB genes of Escherichia coil and Salmonella typhimurium encode the R1 and R2 proteins that together form an active class I ri bonucleotide reductase. Both organisms contain two additional chromoso mal genes, nrdE and nrdF, whose corresponding protein sequences show s ome homology to the products of the genes nrdA and nrdB. When present on a plasmid, nrdE and nrdF together complement mutations in nrdA or n rdB. We have now obtained in nearly homogeneous form the two proteins encoded by the S. typhimurium nrdE and nrdF genes (R1E and R2F). They correspond to the R1 and R2 proteins. Each protein is a homodimer. Tog ether they catalyze the reduction of CDP to dCDP, using dithiothreitol or reduced glutaredoxin, but not thioredoxin, as an electron donor. C DP reduction is strongly stimulated by low concentrations of dATP, pre sumably acting as an allosteric effector. Protein R2F contains an anti ferromagnetically coupled dinuclear iron center and a tyrosyl free rad ical. The E. coli and S. typhimurium chromosome thus have maintained t he information for a potentially active additional class I ribonucleot ide reductase, whose role in vivo is as yet unknown. The allosteric re gulation of this enzyme differs from that of the normally expressed re ductase.