ALLOSTERIC REGULATION OF THE 3RD RIBONUCLEOTIDE REDUCTASE (NRDEF ENZYME) FROM ENTEROBACTERIACEAE

Enterobacteriaceae contain genes for three separate ribonucleotide red uctases: nrdAB code for a class Ia enzyme, active during aerobiosis, n rdDG for a class III enzyme, active during anaerobiosis, and nrdEF for a cryptic class Ib enzyme, The NrdEF enzyme provides the active reduc tase in other, widely different bacteria, Here, we describe the allost eric regulation of the Salmonella, typhimurium NrdEF enzyme, It consis ts of two tightly bound homodimeric proteins, R1E and R2F. Nucleoside triphosphates (ATP, dATP, dGTP, and dTTP) regulate the substrate speci ficity by binding to a single site af the R1E protein (one nucleotide per polypeptide). Regulation is similar to that of the NrdAB enzyme, w ith one major exception: dATP stimulates reduction of CDP (and UDP) un der conditions when dATP strongly inhibits all activity of the NrdAB e nzyme. The nrdA-coded RI protein contains a second binding site for dA TP (and ATP) that controls general enzyme activity, All known R1E prot eins lack tile 50 N-terminal amino acids of R1, and we propose that th e activity site is located ill this area of the protein, The more soph isticated regulation of NrdAB enzymes of eukaryotes provides protectio n against the possibly harmful overproduction of dNTPs.