Involved in the biosynthesis of the antibiotic D-cycloserine (DCS), a cyclic structural analog of D-alanine, used as an antitubercular agent. Catalyzes the addition of hydroxyurea on O-acetyl-L-serine (OAS) to yield O-ureido-L-serine. It prefers sulfide as the second substrate, followed by hydroxyurea, L-homocysteine, and thiosulfate. hydroxyurea + O-acetyl-L-serine = acetate + H(+) + O-ureido-L-serine hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Homotetramer. Belongs to the cysteine synthase/cystathionine beta-synthase family.