Cystathionine gamma-synthase (CGS) catalyses the first committed step of methionine biosynthesis in higher plants. CGS is encoded by the CGS1 gene in Arabidopsis. Stability of CGS1 mRNA is down-regulated in response to methionine application and the exon 1-coding region of CGS1 itself is necessary and sufficient for this regulation. mto1 (for (m) under bare (t) under bar hionine (o) under bar veraccumulation) mutants of Arabidopsis, which carry single amino-acid sequence alterations within CGS1 exon 1, a re deficient in this regulation and overaccumulate methionine. Since CGS1 exon 1 acts in cis during this regulation, we have proposed a model that the regulation occurs during translation of CGS1 mRNA when the nascent polypeptide of CGS and its mRNA are in close proximity. in fact, application of the translation inhibitor cycloheximide abolished this regulation in vivo. This model predicts that the regulation can be reproduced in an in vitro translation system. Studies using the in vitro translation system of wheatgerm extract have indicated that S-adenosylmethionine, a direct metabolite of methionine, is the effector of this regulation. A 5'-truncated RNA species, which is a probable degradation intermediate of CGS1 mRNA in vivo, was also detected in vitro, suggesting that the wheatgerm in vitro translation system reflects the in vivo regulation.