Abstract
The expression of cystathionine γ-synthase (CGS), the key-step enzyme in methionine biosynthesis, is down-regulated in response to S-adenosylmethionine (SAM) at the step of mRNA stability. The first exon of CGS is necessary and sufficient for this regulation, and an amino acid sequence within exon 1 (the MTO1 region) is particularly important. It has been suggested that upon SAM application, the nascent polypeptide of exon 1 induces translation elongation arrest followed by mRNA degradation. A peptidyl-RNA, which is likely due to translation arrest at around the MTO1 region, was detected when GST-tagged CGS exon 1 was translated in vitro in the presence of SAM. Translation down to Ser-94, immediately downstream of the MTO1 region, is required for this translation arrest to occur, and a ribosome stalled at Ser-94 was detected by toeprint analysis. We further identified the RNA moiety of the peptidyl-RNA as tRNASer, decoding the 94th codon.
