Abstract
: The first step of methionine biosynthesis in higher plants, the condensation of O-phosphohomoserine and L-cys-teine forming the thioether cystathionine, is catalyzed by the enzyme cystathionine γ-synthase (CgS). We have isolated a cDNA encoding CgS from a leaf Λ ZAP Il-library of Solanum tuberosum L. The nucleotide and deduced amino acid sequence showed homology to the other CgS sequences from Arabidopsis thalianc., Zea mays, Mesembryanthemum crystailinum and Frogar-ia vesca. The truncated cDNA without putative leader peptide, when cloned into a bacterial expression vector, complemented the E. coil metBi mutant strain LE392. Enzyme activity of the potato CgS and E. coil 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!mei 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.
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