Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway. Also catalyzes the conversion of O-acetylserine (OAS) into cysteine, the last step in the cysteine biosynthesis pathway. methanethiol + O-acetyl-L-homoserine = acetate + H(+) + L-methionine hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from O-acetyl-L-homoserine. Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homotetramer. Belongs to the trans-sulfuration enzymes family.