Cystathionine gamma-synthase (CGS) catalyzes the first committed step of Met biosynthesis in plants. We have previously shown that expression of the gene for CGS is feedback-regulated at the level of mRNA stability, and that the amino acid sequence encoded by the first exon of the CGS gene itself is responsible for the regulation (Chiba, Y., Ishikawa, M., Kijima, F., Tyson, R. H., Kim, J., Yamamoto, A., Nambara, E., Leustek, T., Wallsgrove, R. M., and Naito, S. (1999) Science 286, 1371-1374). To identify the functional region within CGS exon 1, deletion analysis was performed. The results showed that the 41-amino acid region of exon 1 highly conserved among plants is necessary and sufficient for the regulation. Analyses of in vivo and in vitro generated mutations that abolish the regulation identified the functionally important amino acids as 11-13 residues within this conserved region. The importance of these residues was confirmed by deletion analysis within the conserved region. These studies identified the functional region of CGS exon 1 required for the posttranscriptional autoregulation of the CGS gene as (A)RRNCSNIGVAQ(I), with uncertainty of the first and last residues. This sequence is almost perfectly conserved among CGS sequences of higher plants but cannot be found elsewhere in the public databases.