Cloning and characterization of cystathionine γ-synthase from Solanum tuberosum L.

The first step of methionine biosynthesis in higher plants, the condensation of O-phosphohomoserine and L-cysteine forming the thioether cystathionine, is catalyzed by the enzyme cystathionine gamma-synthase (CgS). We have isolated a cDNA encoding CgS from a leaf lambda ZAP II-library of Solanum tuberosum L. The nucleotide and deduced amino acid sequence showed homology to the other CgS sequences from Arabidopsis thaliana, Zea mays, Mesembryanthemum crystallinum and Fragaria vesca. The truncated cDNA without putative leader peptide, when cloned into a bacterial expression vector, complemented the E. coli metB1 mutant strain LE392. Enzyme activity of the potato CgS and E. coli metB were comparable in the auxotrophic background. Transcript levels of CgS in different tissues of potato plants were determined by Northern blot analysis. Expression was found in all tissues with elevated levels in flowers and source leaves. The pattern of gene expression during a day/night period implied light-dependent control of CgS transcription typical for enzymes localized in plastids. The expression of CgS was shown to be light-inducible.