Catalyzes the conversion of O-acetylserine and homocysteine to cystathionine. L-homocysteine + O-acetyl-L-serine = acetate + H(+) + L,L-cystathionine By methionine. Repressed by sulfate and cysteine. Grows very slowly with methionine as sole sulfur source. Belongs to the cysteine synthase/cystathionine beta-synthase family.