Catalyzes the production of cysteine from cystathionine in the reverse transsulfuration pathway for the biosynthesis of sulfur-containing amino acids cysteine and methionine. In this pathway, homocysteine sulfur is converted to cysteine sulfur (Probable). Also has cystathionine beta-lyase and cystathionine gamma-synthase activities in vitro. Cystathionine beta-lyase may be physiological, while cystathionine gamma-synthase activity is not, as the required substrate O-succinyl-L-homoserine(OSH) does not occur naturally in S.cerevisiae (PubMed:8335636). H2O + L,L-cystathionine = 2-oxobutanoate + L-cysteine + NH4(+) Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-homocysteine and L-serine: step 2/2. Homotetramer. Present with 38300 molecules/cell in log phase SD medium. Belongs to the trans-sulfuration enzymes family.