Molecular cloning and functional characterization of cDNAs encoding cysteine synthase and serine acetyltransferase that may be responsible for high cellular cysteine content in Allium tuberosum

The plants belonging to the genus Allium are known to accumulate sulfur-con taining secondary compounds that are derived from cysteine. Here, we report on molecular cloning and functional characterization of two cDNAs that enc ode serine acetyltransferase and cysteine synthase from A. tuberosum (Chine se chive). The cDNA for serine acetyltransferase encodes an open reading fr ame of 289 amino acids, of which expression could complement the lacking of cysE gene for endogenous serine acetyltransferase in Escherichia coli. The cDNA for cysteine synthase encodes an open reading frame of 325 amino acid s, of which expression in the E. coil lacking endogenous cysteine synthase genes could functionally rescue the growth without addition of cysteine. Bo th deduced proteins seem to be localized in cytosol, judging from their pri mary structures. Northern blot analysis indicated that both transcripts acc umulated in almost equal levels in leaves and root of green and etiolated s eedlings of A. tuberosum. The activity of recombinant serine acetyltransfer ase produced from the cDNA was inhibited by L-cysteine, which is the end-pr oduct of the pathway; however, the sensitivity to cysteine (48.7 muM of the concentration for 50% inhibition, IC50) was fairly low compared with that of previously reported serine acetyltransferases (similar to5 muM IC50) fro m Various plants. In A. tuberosum, the cellular content of cysteine was sev eral-fold higher than those in Arabidopsis thaliana and tobacco. This highe r concentration of cysteine in A. tuberosum is likely due to the lower sens itivity of feedback inhibition of serine acetyltransferase to cysteine. (C) 2000 Elsevier Science B.V. All rights reserved.