THE RIBONUCLEOTIDE REDUCTASE SYSTEM OF LACTOCOCCUS-LACTIS - CHARACTERIZATION OF AN NRDEF ENZYME AND A NEW ELECTRON-TRANSPORT PROTEIN

Escherichia coli contains the genetic information for three separate r ibonucleotide reductases. Two of them (class I enzymes), coded by the nrdAB and nrdEF genes, respectively, contain a tyrosyl radical, whose generation requires oxygen, The NrdAB enzyme is physiologically active . The function of the nrdEF gene is not known. The third enzyme (class III), coded by nrdDG, operates during anaerobiosis, The DNA of Lactoc occus lactis contains sequences homologous to the nrdDG genes, Surpris ingly, an nrdD(-) mutant of L. lactis grew well under standard anaerob ic growth conditions. The ribonucleotide reductase system of this muta nt was shown to consist of an enzyme of the NrdEF-type and a small ele ctron transport protein, The coding operon contains the nrdEF genes an d two open reading frames, one of which (nrdH) codes for the small pro tein. The same gene organization is present in E. coli, We propose tha t the aerobic class I ribonucleotide reductases contain two subclasses , one coded by nrdAB, active in E, coli and eukaryotes (class Ia), the other coded by nrdEF, present in various microorganisms (class Ib), T he NrdEF enzymes use NrdH proteins as electron transporter in place of thioredoxin or glutaredoxin used by NrdAB enzymes, The two classes al so differ in their allosteric regulation by dATP.