Pathways of assimilative sulfur metabolism in Pseudomonas putida

Cysteine and methionine biosynthesis was studied in Pseudomonas putida S-31 3 and Pseudomonas aeruginosa PAO1. Both these organisms used direct sulfhyd rylation of O-succinylhomoserine for the synthesis of methionine but also c ontained substantial levels of O-acetylserine sulfhydrylase (cysteine synth ase) activity. The enzymes of the transsulfuration pathway (cystathionine g amma-synthase and cystathionine P-lyase) were expressed at low levels in bo th pseudomonads but were strongly upregulated during growth with cysteine a s the sole sulfur source, In P. aeruginosa, the reverse transsulfuration pa thway between homocysteine and cysteine, with cystathionine as the intermed iate, allows P. aeruginosa to grow rapidly with methionine as the sole sulf ur source. P. putida S-313 also grew well with methionine as the sulfur sou rce, but no cystathionine gamma-lyase, the key enzyme of the reverse transs ulfuration pathway, was found in this species. In the absence of the revers e transsulfuration pathway, P. putida desulfurized methionine by the conver sion of methionine to methanethiol, catalyzed by methionine gamma-lyase, wh ich was upregulated under these conditions. A transposon mutant of P. putid a that was defective in the alkanesulfonatase locus (ssuD) was unable to gr ow with either methanesulfonate or methionine as the sulfur source. We ther efore propose that in P. putida methionine is converted to methanethiol and then oxidized to methanesulfonate. The sulfonate is then desulfonated by a lkanesulfonatase to release sulfite for reassimilation into cysteine.