A. Jordan et al., A 2ND CLASS-I RIBONUCLEOTIDE REDUCTASE IN ENTEROBACTERIACEAE - CHARACTERIZATION OF THE SALMONELLA-TYPHIMURIUM ENZYME, Proceedings of the National Academy of Sciences of the United Statesof America, 91(26), 1994, pp. 12892-12896
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.