CHARACTERIZATION OF ESCHERICHIA-COLI NRDH - A GLUTAREDOXIN-LIKE PROTEIN WITH A THIOREDOXIN-LIKE ACTIVITY PROFILE

Ribonucleotides are converted to deoxyribonucleotides by ribonucleotid e reductases. Either thioredoxin or glutaredoxin is a required electro n donor for class I and II enzymes. Glutaredoxins are reduced by gluta thione, thioredoxins by thioredoxin reductase. Recently, a glutaredoxi n-like protein, NrdH, was isolated as the functional electron donor fo r a NrdEF ribonucleotide reductase, a class Ib enzyme, from Lactococcu s lactis. The absence of glutathione in this bacterium raised the ques tion of the identity of the intracellular reductant for NrdH. Homologu es of NrdH are present in the genomes of Escherichia coli and Salmonel la typhimurium, upstream of the genes for the poorly transcribed nrdEF , separated from it by arm open reading frame (nrdI) coding for a prot ein of unknown function, Overexpression of E. coli NrdH protein shows that it is a functional hydrogen donor with higher specificity for the class Ib (NrdEF) than for the class Ia (NrdAB) ribonucleotide reducta se. Furthermore, this glutaredoxin-like enzyme is reduced by thioredox in reductase and not by glutathione. We suggest that several uncharact erized glutaredoxin-like proteins present in the genomes of organisms lacking GSH, including archae, will also react with thioredoxin reduct ase and be related to the ancestors from which the GSH-dependent gluta redoxins have evolved by the acquisition of a GSH-binding site, We als o show that NrdI, encoded by all nrdEF operons, hats a stimulatory eff ect on ribonucleotide reduction.