The first step of cysteine biosynthesis in bacteria and plants consists in the formation of O-acetylserine catalyzed by serine acetyltransferase (SAT). SAT is highly sensitive to feedback inhibition by cysteine as part of the regulatory circuit of cysteine biosynthesis und thus hampers over-expression and fermentation of cysteine in biotechnological production processes. Since plants contain multiple SAT isoforms with different cysteine feedback sensitivity, this resource was exploited to demonstrate the suitability of plant SATs for the production of cysteine in both bacteria and plants. Three new cDNAs encoding SATs were isolated from Nicotiana tabacum. The catalytic activity of SAT4 was insensitive up to 0.6 mM cysteine. Expression of SAT4 in a newly constructed Escherichia coli host strain without endogenous SAT activity yielded a significant accumulation of cysteine in the culture medium compared to expression of cysteine sensitive SATs in the same strain. The application of a similarly insensitive SAT isoform from A. thaliana demonstrated the suitability of this approach to increase cysteine levels in transgenic tobacco plants.