Abstract
Cystathionine Γ-synthase (CGS) is the key enzyme of Met biosynthesis in higher plants. Expression of the CGS gene is negatively feedback-regulated at the level of mRNA stability in response to S-adenosylmethionine (SAM), the direct metabolite of Met. A short stretch of amino acid sequence (MTO1 region) encoded by the first exon of CGS itself is involved in this regulation. Since the CGS exon 1 acts in cis, we have proposed that this regulation occurs during translation. Accumulation of partial translation products was observed when an RNA carrying GST-tagged CGS exon 1 was translated in vitro in the presence of SAM, suggesting that translation arrest occurs during the regulation. In addition, polysome profiling analysis showed that decay intermediates of the CGS exon 1-mediated mRNA degradation were detected in ribosome fractions, but not in ribosome-free fractions, implying that a ribosome stalls on a decay intermediate of CGS mRNA in vivo.
