An in vivo internal deletion in the N-terminus region of Arabidopsis cystathionine γ-synthase results in CGS expression that is insensitive to methionine

Cystathionine c-synthase (CGS), the first enzyme of methionine biosynthesis in higher plants, plays an important role in the biosynthesis pathway and in regulating methionine metabolism in plant cells. In response to methionine, the expression of this enzyme is regulated via amino acid sequences located in its N-terminal. Here, using reverse transcription PCR and ribonuclease protection analysis, we demonstrate that, in addition to the full-length CGS transcript, a deleted form exists in Arabidopsis. The deleted transcript of CGS that lacks 90 or 87 nt located internally in the regulatory N-terminal region of CGS maintains the reading frame of the protein. Its association with polyribosomes indicates that this deleted form of CGS is translated. In order to study the function of this deleted form of CGS, we overexpressed it in transgenic tobacco plants. We found that the transgenic plants engineered to express only the deleted form of CGS accumulated methionine to a much higher level than those that expressed the full-length CGS. Furthermore, in vitro feeding experiments revealed that the deleted form of CGS is not subject to feedback regulation by methionine, as reported for the full-length transcript. Therefore, although most likely produced from the full-length CGS, the transcript of the deleted form is insensitive to methionine application and its expression may be important for maintaining methionine metabolism even in the presence of a high level of methionine.