Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway (PubMed:6381643, Ref.5). Has also cysteine synthase (O-acetylserine sulfhydrylase) activity in vitro, but seems only be involved in the alternative pathway of methionine biosynthesis in E.nidulans (PubMed:17482430, PubMed:6381643, PubMed:8249501). methanethiol + O-acetyl-L-homoserine = acetate + H(+) + L-methionine Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from O-acetyl-L-homoserine. Oligomer, probably homohexamer. Up-regulated under sulfur limitation and down-regulated by high methionine concentrations. E.nidulans has another enzyme having cysteine synthase (O-acetylserine sulfhydrylase) activity (cysB) that has been shown to be the enzyme of physiological importance in cysteine biosynthesis. Belongs to the trans-sulfuration enzymes family.