5 mM cystine (●), 1 mM homocysteine (○), 1 mM click here methionine (▲) or in the absence of any sulfur source (△). We observed a similar growth for homocysteine and cystathionine, thiosulfate and cystine or sulfide and sulfite. Strain 13 cannot use methionine as sole sulfur source. This is intriguing since methionine can be converted into homocysteine by the SAM recycling pathway involving MtnN and LuxS and further to cysteine via the reverse transulfuration Alpelisib supplier pathway probably encoded by the genes cpe0176 and cpe0177 (Fig. 1). We then tested the ability of strain 13 to grow in minimal medium containing 1 mM homocysteine or 1 mM cystathionine as sole sulfur source. We observed a growth with homocysteine

and cystathionine indicating

the existence of a pathway of homocysteine to cysteine conversion. Cpe0177 shares 51% and 70% identity with MccA, the cystathionine-β-synthase of B. subtilis and C. acetobutylicum, respectively while Cpe0176 is 56% and 70% identical to MccB, the cystathionine-γ-lyase/homocysteine-γ-lyase of the same microorganisms [8, 19]. This strongly suggests that a reverse transsulfuration pathway is present in C. perfringens (Fig. 1) allowing the utilization of homocysteine, a compound that is present in human blood and tissues as an intermediary metabolite [37]. However, we cannot exclude the existence of another homocysteine to cysteine conversion pathway in C. perfringens. The strain 4EGI-1 purchase 13 was unable acetylcholine to grow on sulfate as sole sulfur source according to the lack of the first steps of the sulfate assimilation pathway. By contrast, strain 13 can grow in the presence of sulfite, sulfide or thiosulfate indicating that C. perfringens can synthesize cysteine from these compounds (Fig. 1 and 2). Sulfite is converted into

sulfide by anaerobic sulfite reductases. Two operons, asrABC1 (cpe1438-1440) and asrABC2 (cpe1536-1538) encoding sulfite reductases are present in the genome. In the presence of sulfide and OAS produced by the serine acetyl-transferase (CysE), the OAS-thiol-lyase (CysK) further synthesizes cysteine. We tested the release of sulfide by the strain 13 after growth in the presence of various sulfur sources using lead acetate papers as a trapping agent. We detected high sulfide production after growth in the presence of sulfite due to sulfite reductase activities and to a lesser extent in the presence of thiosulfate. Sulfite and thiosulfate are taken-up by uncharacterized transporters since transporters sharing similarities neither with the CysPWUA system from E. coli [38] nor with the SA1850 permease from S. aureus [17] are present in the genome of C. perfringens. Thiosulfate is probably converted into cysteine using OAS-thiol-lyase activity as observed in E. coli [38]. Finally, C. perfringens was able to grow in the presence of glutathione. The PepT and PepM proteins could be involved in the degradation of this compound to form cysteine (Fig. 1).