Methylotrophy in Mycobacteria: Dissection of the Methanol Metabolism Pathway in Mycobacterium smegmatis

Methylotrophy, the ability to utilize single-carbon (C1) compounds as the sole carbon and energy sources, is only poorly understood in mycobacteria. Both pathogenic and nonpathogenic mycobacteria, including Mycobacterium tuberculosis, are capable of utilizing C1 compounds to meet their carbon and energy requirements, although the precise pathways are not well studied. Here we present a comprehensive study of methylotrophy in Mycobacterium smegmatis. With several genetic knockouts, we have dissected the entire methanol metabolism pathway in M. smegmatis. We show that while methanol dissimilation in M. smegmatis differs from that in other mycobacterial species, the concluding step of CO2 fixation is similar to that in M. tuberculosis. It is therefore both interesting and important to examine mycobacterial physiology in the presence of alternative carbon sources. ABSTRACT The mycobacteria comprise both pathogenic and nonpathogenic bacteria. Although several features related to pathogenicity in various mycobacterial species, such as Mycobacterium tuberculosis, have been studied in great detail, methylotrophy, i.e., the ability of an organism to utilize single-carbon (C1) compounds as the sole source of carbon and energy, has remained largely unexplored in mycobacteria. Reports are available that suggest that mycobacteria, including M. tuberculosis and M. smegmatis, are capable of utilizing alternative C1 compounds to meet their carbon and energy requirements. However, physiological pathways that are functional in mycobacteria to utilize such carbon compounds are only poorly understood. Here we report the identification and characterization of the gene products required for establishing methylotrophy in M. smegmatis. We present N,N-dimethyl-p-nitrosoaniline (NDMA)-dependent methanol oxidase (Mno) as the key enzyme that is essential for the growth of M. smegmatis on methanol. We show that Mno has both methanol and formaldehyde dehydrogenase activities in vitro. Further, M. smegmatis is able to utilize methanol even in the absence of the major formaldehyde dehydrogenase MscR, which suggests that Mno is sufficient to dissimilate methanol and the resulting formaldehyde in vivo. Finally, we show that M. smegmatis devoid of phosphoenolpyruvate carboxykinase, which has been shown to fix CO2 in M. tuberculosis, does not grow on methanol, suggesting that the final step of methanol utilization requires CO2 fixation for biomass generation. Our work here thus forms the first comprehensive report that explores methylotrophy in a mycobacterial species. IMPORTANCE Methylotrophy, the ability to utilize single-carbon (C1) compounds as the sole carbon and energy sources, is only poorly understood in mycobacteria. Both pathogenic and nonpathogenic mycobacteria, including Mycobacterium tuberculosis, are capable of utilizing C1 compounds to meet their carbon and energy requirements, although the precise pathways are not well studied. Here we present a comprehensive study of methylotrophy in Mycobacterium smegmatis. With several genetic knockouts, we have dissected the entire methanol metabolism pathway in M. smegmatis. We show that while methanol dissimilation in M. smegmatis differs from that in other mycobacterial species, the concluding step of CO2 fixation is similar to that in M. tuberculosis. It is therefore both interesting and important to examine mycobacterial physiology in the presence of alternative carbon sources.