Identification and expression of a cDNA encoding cystathionine gamma-synthase in soybean

Methionine is one of the essential amino acids that are synthesized in low amounts in seeds of many agronomically important crop plants such as soybea n. Cystathionine-gamma-synthase (CS; EC 4.2.99.9) is the branch point enzym e leading to methionine synthesis. We isolated a 1023 nucleotide cDNA-encod ing soybean CS from a leaf cDNA library using a 672-nucleotide Arabidopsis cDNA probe. The complete cDNA contains a single open reading frame of 1608 bp that encodes a 536 amino acid protein with a predicted molecular mass of 58 090 Da. The amino terminal portion of the deduced amino acid sequence i s rich in threonine and serine, suggesting the presence of a chloroplast tr ansit peptide (155 amino acids). The soybean CS amino acid sequence shares sequence identity with several CS proteins, Arabidopsis thaliana, Z. mays, M. crystallium, and Escherichia coli. The coding region minus the transit p eptide was cloned in-frame into pUC18. This construct was used to transform and complement an E. coli methionine auxotroph, AB301. Northern analysis r evealed that a 1.9 kb soybean CS mRNA was expressed at the highest level in 8-day light-grown cotyledons and the lowest level in 8-day dark-grown leav es. Southern analysis suggests that the multiple banding patterns may be in dicative of several restriction sites within the genomic sequence or CS may be part of a small gene family. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.