Functional methylthio-alkane reductases are widely distributed for bacterial organic sulfur acquisition

Methylthio-alkane reductase (MAR) is a relative of nitrogenase that cleaves methylthio-alkanes for methanethiol liberation and subsequent methionine biogenesis. For example, dimethylsulfide, the most abundant volatile organic sulfur compound, is cleaved to methanethiol and methane. To date, the only verified MAR enzymes are from Alphaproteobacteria like Rhodospirillum rubrum, despite the long-time observation of gene homologs with potential MAR function present across diverse taxa. Synthesis of MAR genes from uncultivated organisms and expression in R. rubrum reveals that functional MAR enzymes are present throughout the Clostridia, Bacilli, Negativicutes, Fibrobacteria, and Bacteroidia classes of bacteria, which range from terrestrial anoxic environments to gut microbiomes. Further, the functional MAR identification screens enabled prediction and validation that Heliophilum fasciatum natively possesses a functional MAR, directly demonstrating that MAR enzymes exist in Clostridia. Functional MAR enzymes are only observed in one phylogenetic group, Group IV-C, from the larger nitrogen fixation-like family of sequences, indicating that the remaining phylogenetic groups catalyze alternative and, in many cases, unknown reactions. Finally, either the bona fide NifB of R. rubrum, which synthesizes NifB-co, or the homologous MarB associated with each MAR gene cluster can produce an active MAR enzyme, underpinning that nitrogenase-like metallocofactors function at the heart of all MAR systems.

Volatile organic sulfur compounds like dimethylsulfide are ubiquitous and particularly abundant in anoxic terrestrial and gut environments. Further, inorganic sulfur compounds for microbial growth such as sulfate and sulfite can be limited in these environments. Identification of functional methylthio-alkane reductase enzymes from diverse anaerobic bacteria that inhabit anoxic environments reveals that the capacity to utilize volatile organic sulfur compounds and fulfill cellular sulfur requirements is more widespread than previously considered. In addition to catalytic components, MarHDK, either the associated maturase, MarB, or the canonical nitrogenase maturase, NifB, was required, underpinning that marBHDK is the minimal gene set for methylthio-alkane reductase across bacteria and that, in general, methylthio-alkane reductases utilize nitrogenase-like cofactors for catalysis.