Light and sucrose up-regulate the expression level of Arabidopsis cystathionine γ-synthase, the key enzyme of methionine biosynthesis pathway.

The sulfur-containing essential amino acid methionine controls the level of important metabolites and processes in plants. In addition, methionine levels limit the nutritional quality of many crop plants. The level of methionine is regulated mainly by cystathionine γ-synthase (CGS), the first enzyme committed to its biosynthesis. Within our efforts to reveal factors that regulate CGS and methionine content in plants, we have analyzed how light regulates the transcript and protein level of Arabidopsis CGS (AtCGS). The expression of AtCGS is up-regulated in the light and reduced in the dark, independent of the diurnal cycle. Using tobacco plants overexpressing AtCGS, we have found that the light sensitive motives of the AtCGS gene are found within the coding sequence of AtCGS and not in its promoter, terminator or the untranslated regions of the gene. Sucrose can partially mimic the effect of light in dark grown plants while the addition of nitrogen and sulfur sources does not have any effect. The kinetics of the changes in the expression level of AtCGS suggest that its level can be maintained during extended darkness, or even when the sucrose content is reduced, such as during abiotic stresses. The up-regulation of AtCGS by light is in agreement with previous studies showing that other enzymes regulating the level of the carbon/amino skeleton and the sulfur group of Met, are up-regulated by light. The results indicate that light and dark participate in the regulation of the carbon/amino skeleton flux in the synthesis of amino acids of the aspartate family.