Expression of a bacterial serine acetyltransferase in transgenic potato plants leads to increased levels of cysteine and glutathione

The coding sequence of the wild-type, cys-sensitive, cysE gene from Escheri chia coli, which encodes an enzyme of the cysteine biosynthetic pathway, na mely serine acetyltransferase (SAT, EC 2.3.1.30), was introduced into the g enome of potato plants under the control of the cauliflower mosaic virus 35 S promoter. In order to target the protein into the chloroplast, cysE was t ranslationally fused to the 5'-signal sequence of rbcS from Arabidopsis tha liana. Transgenic plants showed a high accumulation of the cysE mRNA. The c hloroplastic localisation of the E. coli SAT protein was demonstrated by de termination of enzymatic activities in enriched organelle fractions. Crude leaf extracts of these plants exhibited up to 20-fold higher SAT activity t han those prepared from wild-type plants. The transgenic potato plants expr essing the E. coli gene showed not only increased levels of enzyme activity but also exhibited elevated levels of cysteine and glutathione in leaves. Both were up to twofold higher than in control plants. However, the thiol c ontent in tubers of transgenic lines was unaffected. The alterations observ ed in leaf tissue had no effect on the expression of O-acetylserine(thiol)- lyase, the enzyme which converts O-acetylserine, the product of SAT, to cys teine. Only a minor effect on its enzymatic activity was observed. In concl usion, the results presented here demonstrate the importance of SAT in plan t cysteine biosynthesis and show that production of cysteine and related su lfur-containing compounds can be enhanced by metabolic engineering.