NrdI

nrdI is a bacterial gene that encodes a flavodoxin-like protein named NrdI. It is commonly found in organisms that possess ribonucleotide reductase of class Ib (with subunits NrdE and NrdF) and is often located in the same genomic region as these RNR genes. The NrdI protein binds flavin mononucleotide (FMN) and adopts the characteristic flavodoxin fold, enabling it to participate in cellular redox chemistry.

Functionally, NrdI acts as an electron-transfer partner for the NrdF subunit of class Ib ribonucleotide reductase. By cycling FMN between its oxidized and reduced states, NrdI supplies the reducing equivalents necessary for the maturation and activation of the metallo-cofactor in NrdF that enables the conversion of ribonucleotides to deoxyribonucleotides. In this way, NrdI supports DNA precursor synthesis, particularly under conditions where the cell relies on class Ib RNRs, such as oxidative stress, iron limitation, or aerobic growth in certain bacteria.

Genomic and evolutionary context show that nrdI is widely distributed among Gram-positive bacteria and is frequently cosegregated with nrdE and nrdF in operons or gene clusters. While not essential in all organisms, NrdI is considered important for robust RNR activity under challenging environmental conditions and can influence growth and survival when ribonucleotide reduction is required.

Structural notes indicate that NrdI proteins are typically small cytosolic flavoproteins (~180–210 amino acids) that coordinate FMN and interact with NrdF to facilitate cofactor formation and function of the class Ib ribonucleotide reductase.