Insights into the sulfur metabolism of Chlorobaculum tepidum by label-free quantitative proteomics.

Abstract

Chlorobaculum tepidum is an anaerobic green sulfur bacterium which oxidizes sulfide, elemental sulfur, and thiosulfate for photosynthetic growth. It can also oxidize sulfide to produce extracellular S0 globules, which can be further oxidized to sulfate and used as an electron donor. Here, we performed label-free quantitative proteomics on total cell lysates prepared from different metabolic states, including a sulfur production state (10h post-incubation [PI]), the beginning of sulfur consumption (20h PI), and the end of sulfur consumption (40h PI), respectively. We observed an increased abundance of the sulfide:quinone oxidoreductase (Sqr) proteins in 10h PI indicating a sulfur production state. The periplasmic thiosulfate-oxidizing Sox enzymes and the dissimilatory sulfite reductase (Dsr) subunits showed an increased abundance in 20h PI, corresponding to the sulfur-consuming state. In addition, we found that the abundance of the heterodisulfide-reductase and the sulfhydrogenase operons was influenced by electron donor availability and may be associated with sulfur metabolism. Further, we isolated and analyzed the extracellular sulfur globules in the different metabolic states to study their morphology and the sulfur cluster composition, yielding 58 previously uncharacterized proteins in purified globules. Our results show that C. tepidum regulates the cellular levels of enzymes involved in sulfur metabolism in response to the availability of reduced sulfur compounds.