How a methanogen assimilates sulfate: Structural and functional elucidation of the complete sulfate-reduction pathway

Abstract

By growing on sulfate as the sole source of sulfur, Methanothermococcus thermolithotrophicus breaks a dogma: the ancient metabolic pathways methanogenesis and sulfate-reduction should not co-occur in one organism due to toxic intermediates and energetic barriers. Using a complementary approach of physiological, biochemical, and structural studies, we provide a snapshot of the complete sulfate-reduction pathway of the methanogenic archaeon. While the first two reactions proceed via an ATP-sulfurylase and APS-kinase, common to other organisms, the further steps are catalysed by non-canonical enzymes. 3'-phosphoadenosine-5'-phosphosulfate (PAPS) released by the APS-kinase is converted into sulfite and 3'-phosphoadenosine-5'-phosphate (PAP) by a new class of PAPS-reductase that shares high similarity with the APS-reductases involved in dissimilatory sulfate-reduction. The generated PAP is efficiently hydrolysed by a PAP-phosphatase that was likely derived from an RNA exonuclease. Finally, the F420-dependent sulfite-reductase converts sulfite to sulfide for cellular assimilation. While metagenomic and metatranscriptomic studies suggest that genes of the sulfate-reduction pathway are present in various methanogens, M. thermolithotrophicus uses a distinct way to assimilate sulfate. We propose that its entire sulfate-assimilation pathway was derived from a mix-and-match strategy in which the methanogen acquired assimilatory and dissimilatory enzymes from other microorganisms and shaped them to fit its physiological needs.

Differences in the regulation mechanisms of the glutamine synthetase from methanogenic archaea unveiled by structural investigations

“…Cells were grown in a minimal medium as described in Jespersen et al 44 . Anaerobic cultivation of M. thermolithotrophicus was performed in a fermenter with NH4Cl and SO4 2as described in Jespersen et al 2022 45 with slight modifications, or in a fermenter with N2 and SO4 2as described in Maslać et al 2022 40 . We did not observe dramatic changes in the expression of GS nor its properties between both cultivation processes.…”