Chlorobaculum (Cba.) tepidum is a green sulfur bacterium that oxidizes sulfide, 
elemental sulfur, and thiosulfate for photosynthetic growth. To gain insight 
into the sulfur metabolism, the proteome of Cba. tepidum cells sampled under 
different growth conditions has been quantified using a rapid gel-free, 
filter-aided sample preparation (FASP) protocol with an in-solution isotopic 
labeling strategy. Among the 2245 proteins predicted from the Cba. tepidum 
genome, approximately 970 proteins were detected in unlabeled samples, whereas 
approximately 630-640 proteins were detected in labeled samples comparing two 
different growth conditions. Wild-type cells growing on thiosulfate had an 
increased abundance of periplasmic cytochrome c-555 and proteins of the 
periplasmic thiosulfate-oxidizing SOX enzyme system when compared with cells 
growing on sulfide. A dsrM mutant of Cba. tepidum, which lacks the dissimilatory 
sulfite reductase DsrM protein and therefore is unable to oxidize sulfur 
globules to sulfite, was also investigated. When compared with wild type, the 
dsrM cells exhibited an increased abundance of DSR enzymes involved in the 
initial steps of sulfur globule oxidation (DsrABCL) and a decreased abundance of 
enzymes putatively involved in sulfite oxidation (Sat-AprAB-QmoABC). The results 
show that Cba. tepidum regulates the cellular levels of enzymes involved in 
sulfur metabolism and other electron-transferring processes in response to the 
availability of reduced sulfur compounds.