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).