Nascent peptide-mediated translation elongation arrest of Arabidopsis thaliana CGS1 mRNA occurs autonomously.

The Arabidopsis thaliana CGS1 gene encodes cystathionine gamma-synthase, the first committed enzyme of methionine biosynthesis in higher plants. Expression of CGS1 is feedback-regulated at the step of mRNA degradation in response to S-adenosyl-L-methionine (AdoMet). A short stretch of amino acid sequence, termed the MTO1 region, encoded within the first exon of CGS1 itself acts in cis in the regulation. In vitro analyses using wheat germ extract (WGE) revealed that AdoMet induces temporal translation arrest of CGS1 mRNA prior to mRNA degradation. This translational pausing occurs immediately downstream of the MTO1 region and is mediated by the nascent MTO1 peptide. In order to elucidate further the nature of this unique regulatory mechanism, we have examined whether a non-plant system also contains the post-transcriptional regulation activity. Despite the fact that mammals do not carry cystathionine gamma-synthase, AdoMet was able to induce the MTO1 sequence-dependent translation elongation arrest in rabbit reticulocyte lysate (RRL) in a similar manner to that observed in WGE. This result suggests that MTO1 peptide-mediated translation arrest does not require a plant-specific factor and rather most probably occurs via a direct interaction between the nascent MTO1 peptide and the ribosome that has translated it. In contrast, decay intermediates of CGS1 mRNA normally observed upon induction of CGS1 mRNA decay in plant systems were not detected in RRL, raising the possibility that CGS1 mRNA degradation involves a plant-specific mechanism.