NxrB

nxrB is a gene encoding the beta subunit of the nitrite oxidoreductase (NXR) enzyme complex found in many nitrite-oxidizing bacteria. NXR catalyzes the oxidation of nitrite (NO2-) to nitrate (NO3-) and couples this reaction to the electron transport chain, contributing to energy conservation in chemolithoautotrophic nitrification.

Genomic context and distribution: nxrB is commonly located in the nxrAB operon and is frequently co-transcribed with nxrA, which encodes the other large subunit. Orthologs of nxrB are found across diverse nitrite-oxidizing bacteria, including genera such as Nitrobacter and Nitrospira, with sequence variation reflecting phylogenetic diversity.

Protein features and function: The NxrB protein is regarded as the electron-transfer subunit of the NXR complex, working in tandem with NxrA to enable nitrite oxidation. It is thought to contain prosthetic groups that facilitate electron transfer to the respiratory chain; the exact cofactors and molecular organization can vary among species and remain an area of ongoing study.

Ecology, regulation, and applications: Expression of nxrB (and nxrA) is influenced by the presence of nitrite and by oxygen conditions, with regulation differing among taxa. Because nitrite-oxidizing bacteria are key players in the nitrogen cycle, nxrB sequences are widely used in environmental surveys as molecular markers for nitrite-oxidizer activity and diversity. In engineered systems such as wastewater treatment, understanding nxrB aids in monitoring and optimizing nitrification processes.