A nitrogenase-like enzyme is involved in the novel anaerobic assimilation pathway of a sulfonate, isethionate, in the photosynthetic bacteriumRhodobacter capsulatus

Abstract

SummaryProkaryotes contribute to the global sulfur cycle by using diverse sulfur compounds as sulfur sources or electron acceptors. Here we report that a nitrogenase-like enzyme (NFL) and a radical SAM enzyme (RSE) are involved in the novel anaerobic assimilation pathway of a sulfonate, isethionate, in the photosynthetic bacteriumRhodobacter capsulatus. ThenflHDKgenes for NFL are localized at a locus containing genes for known sulfonate metabolism in the genome. A genenflBencoding an RSE is present just upstream ofnflH, forming a small gene clusternflBHDK. Mutants lacking anynflBHDKgenes lost the ability to grow with isethionate as the sole sulfur source under anaerobic photosynthetic conditions, indicating that all four NflBHDK proteins are essential for the isethionate assimilation pathway. Heterologous expression of theislABgenes encoding a known isethionate lyase that degrades isethionate to sulfite and acetaldehyde restored the isethionate-dependent growth of a mutant lackingnflDK, indicating that the enzyme encodingnflBHDKis involved in an isethionate assimilation reaction to release sulfite. Furthermore, heterologous expression ofnflBHDKandssuCABencoding an isethionate transporter in the closely related speciesR. sphaeroides, which does not havenflBHDKand cannot grow with isethionate as the sole sulfur source, conferred isethionate-dependent growth ability to this species. We propose to renamenflBHDKasisrBHDK(isethionate reductase). TheisrBHDKgenes are widely distributed among various prokaryote phyla. Discovery of the isethionate assimilation pathway by IsrBHDK provides a missing piece for the anaerobic sulfur cycle and for understanding the evolution of ancient sulfur metabolism.ImportanceNitrogenase is an important enzyme found in prokaryotes that reduces atmospheric nitrogen to ammonia and plays a fundamental role in the global nitrogen cycle. It has been noted that nitrogenase-like enzymes (NFLs), which share an evolutionary origin with nitrogenase, have evolved to catalyze diverse reactions such as chlorophyll biosynthesis (photosynthesis), coenzyme F430biosynthesis (methanogenesis), and methionine biosynthesis. In this study, we discovered that an NFL with unknown function in the photosynthetic bacteriumRhodobacter capsulatusis a novel isethionate reductase (Isr), which catalyzes the assimilatory degradation of isethionate, a sulfonate, releasing sulfite used as the sulfur source under anaerobic conditions. Isr is widely distributed among various bacterial phyla, including intestinal bacteria, and is presumed to play an important role in sulfur metabolism in anaerobic environments such as animal guts and microbial mats. This finding provides a clue for understanding ancient metabolism that evolved under anaerobic environments at the dawn of life.