RESEARCH TOPICS The regulation of assimilatory sulfate reduction and biosynthesis of cysteine is investigated using the favorite plant of molecular biologists, Arabidopsis thaliana, a member of the Brassicaceae family. Cysteine is of great importance for human and animal nutrition. It is the precursor of donor for all compounds containing reduced sulfur in the cell. Sulfur is indispensable for proteins (methionine, disulfide bridges), electron transport (Fe/S clusters), redox control (glutathione) and cofactors (acetyl coenzyme A, biotin, thiamine, lipoic acid). In plants it additionally contributes to plant defense and secondary metabolism. Reduced sulfur is therefore an indispensable factor for life. Plants as photoautotrophic organisms are, directly or indirectly, the exclusive source of cysteine for animal life.