<p> This model discriminates cysteine synthase A (CysK) and cysteine synthase B (CysM) from cystathionine beta-synthase, a protein found primarily in eukaryotes and carrying a C-terminal CBS domain lacking from this protein. Bacterial proteins lacking the CBS domain but otherwise showing resemblance to cystathionine beta-synthases and considerable phylogenetic distance from known cysteine synthases were excluded. </p> <p>Cysteine synthase (O-acetylserine (thiol)-lyase, <db_xref db="EC" dbkey="2.5.1.47"/>) is the enzyme responsible for the formation of cysteine from O-acetyl-serine and hydrogen sulphide with the concomitant release of acetic acid. In bacteria such two forms of the enzyme are known (genes cysK and cysM). CysK differs from CysM in that it can also use sulphide instead of thiosulphate, to produce cysteine instead of cysteine thiosulphonate. </p> Cysteine synthase A