OASTL does not incorporate sulfur from TMTM into cysteine
Sulfate is normally reduced to sulfide, which is as substrate for OASTLs to form cysteine. Cysteine is further converted to GSH, methionine or other sulfur-containing metabolites. TMTM is an organosulfide, containing 3 sulfide groups. We tested if plants can synthesize cysteine using TMTM as substrate. An OASTL activity assay was conducted by incubating total protein extract from wild-type A. thaliana(ecotype Col-0 ) leaves with OAS and either Na2S or TMTM as substrate. Cysteine production was only observed when Na2S was used as substrate (Figure 7). In another experimental setup, total protein extract, OAS, Na2S and TMTM were incubated in the same reaction tube. Also under this condition, cysteine was produced, which indicates that OASTL activity was not hindered by TMTM. We conclude that TMTM is not a direct substrate for sulfur incorporation into cysteine by OASTL under our experimental conditions (Figure 7).