The flavodoxin FldA activates the class Ia ribonucleotide reductase of Campylobacter jejuni

Campylobacter jejuni is a microaerophilic zoonotic pathogen with an atypical respiratory Complex I that oxidizes a flavodoxin (FldA) instead of NADH. FldA is essential for viability and is reduced via pyruvate and 2‐oxoglutarate oxidoreductases (POR/OOR). Here, we show that FldA can also be reduced by FqrB (Cj0559), an NADPH:FldA reductase. An fqrB deletion mutant was viable but displayed a significant growth defect. FqrB is related to flavoprotein reductases from Gram‐positive bacteria that can reduce NrdI, a specialized flavodoxin that is needed for tyrosyl radical formation in NrdF, the beta subunit of class 1b‐type (Mn) ribonucleotide reductase (RNR). However, C. jejuni possesses a single class Ia‐type (Fe) RNR (NrdAB) that would be expected to be ferredoxin dependent. We show that CjFldA is an unusually high potential flavodoxin unrelated to NrdI, yet growth of the fqrB mutant, but not the wild‐type or a complemented strain, was stimulated by low deoxyribonucleoside (dRNS) concentrations, suggesting FldA links FqrB and RNR activity. Using purified proteins, we confirmed the NrdB tyrosyl radical could be regenerated in an NADPH, FqrB, and FldA dependent manner, as evidenced by both optical and electron paramagnetic resonance (EPR) spectroscopy. Thus, FldA activates RNR in C. jejuni, partly explaining its essentiality. Our work reveals a new function for the Campylobacter jejuni flavodoxin (FldA) in the formation of a tyrosyl radical (Y●) in the DNA synthesis enzyme ribonucleotide reductase (RNR). FldA and RNR are essential for viability and represent possible drug targets for this zoonotic food‐borne pathogen, which is the commonest cause of bacterial gastroenteritis.