L-homocysteine + L-serine = H2O + L,L-cystathionine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-homocysteine and L-serine: step 1/2. Present with 41900 molecules/cell in log phase SD medium. Belongs to the cysteine synthase/cystathionine beta-synthase family.