Nitrogen is an essential ingredient of life that is represented in the biosphere by a diverse set of chemical species exhibiting a wide range of oxidation states. The interconversion between these forms underpins huge fluxes through the biosphere in what is known as the nitrogen cycle. Nitrification is one branch of the nitrogen cycle currently understood to only be carried out by a limited number of specialist microbes, who oxidise either ammonia (NH3) or nitrite (NO2-) as a primary energy source to support growth. Here, we present evidence that the soil bacterium Mycobacterium smegmatis is capable of nitrite oxidation, the first example of a species from the phylum Actinobacteria that can do so. Furthermore, instead of consuming nitrite in order to support growth, we found that nitrite is oxidised during carbon starvation to support persistence. We have demonstrated using oxygen electrode measurements in conjunction with genetic knockouts and chemical inhibitors that nitrite oxidation is a respiratory process dependant on a nitrite oxidoreductase (NXR). Due to the high reduction potential of nitrite, we hypothesise that NXR forms novel interactions with the terminal oxidases of the respiratory chain and are testing this with an inducible CRISPRi transcriptional knockdown system alongside respiratory inhibitors. We are using shotgun proteomics and qRT-PCR to delineate the patterns of NXR expression during persistence. The NXR of M. smegmatis was originally annotated as a respiratory nitrate reductase (NAR) that catalyses the reverse reaction. NAR genes are widespread among Actinobacteria, suggesting that nitrite oxidation could be a prevalent strategy to support persistence in this phylum. Accordingly, we are testing other Actinobacteria with putative NAR genes for their ability to oxidise nitrite.