Three methylthio-alkane reductase homologs in Rhodopseudomonas palustris have distinct substrate specificities

Bacteria play a critical role in the global sulfur cycle by using enzymes capable of cleaving strong bonds such as C-S. Most described enzymes require oxygen for their activity, but recent work has uncovered anaerobic alternatives. Among these are the methylthio alkane reductases (Mar), a group of nitrogenase-like enzymes that reduce volatile organic sulfur compounds (VOSCs) in the absence of oxygen. The purple non-sulfur bacterium Rhodopseudomonas palustris encodes three Mar homologs, providing a unique system to study their distinct functions. Here, we characterize the activity of these enzymes using genomic (RB-TnSeq) and transcriptomic (RNA-Seq) approaches to assess the essentiality and expression patterns of the different Mar enzymes under sulfate limitation. Growth assays with knockout strains supplied with different VOSCs, combined with measuring the specific reaction by-product, revealed that Mar1 preferentially acts on shorter-chain substrates. Our findings highlight the functional diversification of Mar enzymes and expand our understanding of their role in anaerobic sulfur metabolism. Overall design: RNA profilling of WT Rhodopseudomonas palustris CGA009 cells grown in the presence and absence of sulfate (absence in this case being < 200µM of sulfate) and 1mM MT-EtOH (a volatile organic sulfur compound)