NrdI, a flavodoxin involved in maintenance of the diferric-tyrosyl radical cofactor in Escherichia coli class Ib ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is essential in all organisms. Class I RNRs consist of two homodimeric subunits: α2 and β2. The subunit contains the site of nucleotide reduction, and the β subunit contains the essential diferric-tyrosyl radical (Y•) cofactor. Escherichia coli contains genes encoding two class I RNRs (Ia and Ib) and a class III RNR, which is active only under anaerobic conditions. Its class Ia RNR, composed of NrdA (α) and NrdB (β), is expressed under normal aerobic growth conditions. The class Ib RNR, composed of NrdE (α) and NrdF (β), is expressed under oxidative stress and iron-limited growth conditions. Our laboratory is interested in pathways of cofactor biosynthesis and maintenance in class I RNRs and modulation of Y• levels as a means of regulating RNR activity. Our recent studies have implicated a [2Fe2S]-ferredoxin, YfaE, in the NrdB diferric-Y• maintenance pathway and possibly in the biosynthetic and regulatory pathways. Here, we report that NrdI is a flavodoxin counterpart to YfaE for the class Ib RNR. It possesses redox properties unprecedented for a flavodoxin (E _ox/sq264 ± 17 mV and E_sq/hq = -255 ± 17 mV) that allow it to mediate a two-electron reduction of the diferric cluster of NrdF via two successive one-electron transfers. Data presented support the presence of a distinct maintenance pathway for NrdEF, orthogonal to that for NrdAB involving YfaE.