Abstract
ABSTRACT5′-Methyl-thioadenosine (MTA) is a dead-end, sulfur-containing metabolite and cellular inhibitor that arises from S-adenosyl-l-methionine-dependent reactions. Recent studies have indicated that there are diverse bacterial methionine salvage pathways (MSPs) for MTA detoxification and sulfur salvage. Here, via a combination of gene deletions and directed metabolite detection studies, we report that under aerobic conditions the facultatively anaerobic bacterium Rhodopseudomonas palustris employs both an MTA-isoprenoid shunt identical to that previously described in Rhodospirillum rubrum and a second novel MSP, both of which generate a methanethiol intermediate. The additional R. palustris aerobic MSP, a dihydroxyacetone phosphate (DHAP)-methanethiol shunt, initially converts MTA to 2-(methylthio)ethanol and DHAP. This is identical to the initial steps of the recently reported anaerobic ethylene-forming MSP, the DHAP-ethylene shunt. The aerobic DHAP-methanethiol shunt then further metabolizes 2-(methylthio)ethanol to methanethiol, which can be directly utilized by O-acetyl-l-homoserine sulfhydrylase to regenerate methionine. This is in contrast to the anaerobic DHAP-ethylene shunt, which metabolizes 2-(methylthio)ethanol to ethylene and an unknown organo-sulfur intermediate, revealing functional diversity in MSPs utilizing a 2-(methylthio)ethanol intermediate. When MTA was fed to aerobically growing cells, the rate of volatile methanethiol release was constant irrespective of the presence of sulfate, suggesting a general housekeeping function for these MSPs up through the methanethiol production step. Methanethiol and dimethyl sulfide (DMS), two of the most important compounds of the global sulfur cycle, appear to arise not only from marine ecosystems but from terrestrial ones as well. These results reveal a possible route by which methanethiol might be biologically produced in soil and freshwater environments.
Highlights
5=-Methyl-thioadenosine (MTA) is a dead-end, sulfur-containing metabolite and cellular inhibitor that arises from S-adenosyl-L-methionine-dependent reactions
R. palustris contains multiple aerobic methionine salvage pathways linked to methanethiol production
In R. rubrum, aerobic and anaerobic metabolism of MTA leads to the formation of volatile methanethiol by virtue of the MTA-isoprenoid shunt methionine salvage pathways (MSPs) (Fig. 1; blue) (14, 23)
Summary
5=-Methyl-thioadenosine (MTA) is a dead-end, sulfur-containing metabolite and cellular inhibitor that arises from S-adenosyl-L-methionine-dependent reactions. Given that many organisms encounter sulfur-limiting environments, further recycling of MTA to regenerate a usable sulfur source, typically L-methionine, is necessary to maintain proper cellular sulfur pools This is underpinned by the fact that most eukaryotes possess the universal methionine salvage pathway (MSP), referred to as the canonical MSP or, in plant systems, the Yang cycle (Fig. 1; black). The canonical pathway consists of six enzymatic steps: nucleosidase/phosphorylase (MtnP), isomerase (MtnA), dehydratase (MtnB), enolase/phosphatase (MtnC), dioxygenase (MtnD), and transaminase (MtnE) (Fig. 1; enzymes D to F and enzymes I to K) (13) This results in the conversion of MTA to adenine, formate, and L-methionine at the expense of inorganic phosphate, molecular oxygen, and a suitable amino acid as an amine donor. The products and oxygen requirements are the same as those of the canonical MSP
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