Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway (PubMed:11844756, Ref.4, PubMed:18050920). Can also use dimethyldisulfide and methanethiol as reduced sulfur sources, leading to the direct formation of methionine (PubMed:20798582). Has weak cystathionine gamma-synthase activity (PubMed:18050920). hydrogen sulfide + O-acetyl-L-homoserine = acetate + L-homocysteine methanethiol + O-acetyl-L-homoserine = acetate + H(+) + L-methionine Inhibited by methionine and cystathionine. kcat is 28.0 sec(-1) with O-acetyl-L-homoserine as substrate for the O-acetyl-L-homoserine sulfhydrylase activity. kcat is 37.3 sec(-1) with Na(2)S as substrate. kcat is 7.0 sec(-1) with O-acetyl-L-homoserine as substrate for the cystathionine gamma-synthase activity. Optimum pH is 7.0. Optimum temperature is 37-42 degrees Celsius. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from O-acetyl-L-homoserine: step 1/1. Homohexamer. Activity is maximal during the early stationary phase. Expression is repressed by methionine. Disruption of the metY gene is not enough to lead to methionine auxotrophy, but the metB-metY double mutant is unable to grow on a minimal medium lacking supplemental methionine (PubMed:11844756). Deletion of the gene results in methionine auxotrophy on methanethiol or dimethyldisulfide as sole sulfur source (PubMed:20798582). Belongs to the trans-sulfuration enzymes family.