Catalyzes the removal of elemental sulfur atoms from cysteine to produce alanine. Seems to participate in the biosynthesis of the nitrogenase metalloclusters by providing the inorganic sulfur required for the Fe-S core formation. [sulfur carrier]-H + L-cysteine = [sulfur carrier]-SH + L-alanine Homodimer. Belongs to NIF2 gene cluster which is expressed in vegetative cells and heterocysts under anaerobic conditions only. This second system appears to serve as an auxiliary system that does not suppress expression of the first system. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. NifS/IscS subfamily.