Serine acetyltransferase (SAT) catalyzes the first step of cysteine synthesis in microorganisms and higher plants. Rate-limiting step, abundant O-Acetylserine (thiol) Lyase drive to Cysteine. In the crystal, SAT forms a hexamer with each monomer containing a cysteine molecule in the potential catalytic site. The monomer structure is composed of two distinctive domains, a beta-helix domain and an alpha helix domain. Cysteine molecules were found between two monomers in the alpha-helical regions.