5 mM cystine (1, 3, 5 and 7) or 1

mM homocysteine (2, 4, 6 and 8). MccB belongs to a family of PLP-dependent enzymes with O-acyl-homoserine γ-synthase, cystathionine β-lyase, cystathionine γ-lyase, methionine γ-lyase or O-acyl-homoserine CYT387 thiol-lyase activity [47]. Several elements strongly support that Cpe0176/MccB is involved in reverse transsulfuration: i) MccB is more similar to characterized cystathionine γ-lyases of B. subtilis and C. acetobutylicum than to the other members of this family; ii) MccB has an homocysteine γ-lyase activity associated with cystathionine-γ-lyase activity [8]; iii) mccB is in operon with mccA encoding a cystathionine-β-synthase-type enzyme and ubiG, encoding a SAM-dependent methyl-transferase as observed in several firmicutes [8, 9, 19]; iv) C. perfringens can grow in the presence of homocysteine as sole sulfur source; v) the expression of the ubiG operon is induced by cysteine depletion via a cysteine specific T-box element as expected for a cysteine biosynthetic pathway. In addition to its control by a T-box regulatory element, the ubiG operon also belongs to the VirR and VirX regulons. Interestingly, Saracatinib we showed that another member of the VirR and VirX regulons, the pfoA gene encoding perfringolysin O [24, 27], was regulated in response to cysteine availability. pfoA expression increased 3- (transcriptome) and 6-fold (qRT-PCR)

in the presence of cystine compared with homocysteine (Table 1). However, it seems unlikely that the effect of cysteine is mediated by the VirR/VirS system since cysteine does not induce the expression of other VirR/VirS-activated Tideglusib genes [48]. Regulation of other genes involved in sulfur metabolism by cysteine availability An S-box motif is located upstream of two genes that were derepressed during cysteine depletion in the transcriptome study: the metK gene encoding the SAM-synthase and the cpe2317 gene (metT) encoding a potential methionine transporter [9] (Fig. 1). Cpe2317/MetT is an antiporter of the NhaC superfamily that is

present in B. cereus, S. aureus, C. botulinum and C. tetani with S-boxes preceding the corresponding genes [9]. Quantitative RT-PCR experiments confirmed that the quantity of the metK transcript was 14-fold higher in the presence of homocysteine than in the presence of cystine. This suggested that the concentration of SAM is limited during growth with homocysteine. We were unable to detect methionine (Fig. 3) suggesting a low concentration for this amino acid. We also failed to reproducibly determine the SAM concentration probably due to the weak stability of this compound. In this study, we identified additional genes that could participate in sulfur metabolism. We observed an increased transcription of cpe1371 in the presence of homocysteine (3.3-fold in transcriptome and 5-fold in qRT-PCR experiments).