Salicylic acid restored the growth of trpE2, entC, entD and (entDtrpE2) mutants, but only to a limited extent when added up to 5 μg mL−1 in the medium. Hence, the mutants are not strict auxotrophs of salicylic acid, but this may be because the deleted proteins also have an (unproven) involvement in the conversion of salicylic acid into both mycobactin and carboxymycobactin. Interestingly, although neither mycobactin nor carboxymycobactin individually restored the growth of the knockout mutants, DAPT they did so together (Fig. 3). This suggests that carboxymycobactin may be more important
in iron metabolism than hitherto considered in spite of it being a minor siderophore in this organism (Ratledge & Ewing, 1996). The results also indicate that mycobactin is not converted to carboxymycobactin and vice versa as then there would have
been no enhancement of growth when both siderophores were added together. In M. smegmatis, salicylic acid is produced from the shikimic acid pathway via chorismic and isochorismic acids (Marshall & Ratledge, 1972). In P. aeruginosa, genetic and experimental evidences indicate that pchA and pchB genes encode ICS and isochorismate pyruvate-lyase, respectively, catalyzing in turn the conversion of chorismate to isochorismate and then isochorismate to pyruvate plus salicylate for the biosynthesis of pyochelin (Serino et al., 1995; Gaille et al., 2002). When the purified ICS from P. aeruginosa was examined for salicylate synthesis, there was no reaction in vitro (Gaille Selleck GSK2118436 et al., 2003); additionally, in vivo, PchA did not display salicylate synthase activity. An entC mutant of E. coli carrying only the pchA gene also failed to produce salicylate, but when the same mutant had both pchA and pchB genes, salicylate synthesis took place (Serino et al., 1995). Hence, organisms that have no PchB protein homolog can carry out the direct learn more conversion
of chorismate to salicylate, for example MbtI of M. tuberculosis, Irp-9 of Y. enterocolitica and YbtS of Y. pestis (Gehring et al., 1998; Quadri et al., 1998). This proposition was supported by studies where native and purified protein MbtI from M. tuberculosis was shown, not to function as ICS like PchA, but instead acted as a salicylate synthase like Irp-9 (Harrison et al., 2006). In Yersinia spp., which again synthesizes salicylic acid for the production of yersiniabactin, the conversion of chorismic acid to isochorismic acid and then to salicylic acid is by a single gene product acting as a bifunctional salicylate synthase (Kerbarh et al., 2005) as was the case in M. tuberculosis (Harrison et al., 2006). To elucidate genes for salicylate biosynthesis in M. smegmatis, we generated knockout mutants of the likely key genes trpE2, entC and entD by targeted mutagenesis. From the enzymatic analysis of salicylic acid biosynthesis by CFEs from the various mutants of M.