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 Inhibited by equimolar concentrations of p-chloromercuribenzoic acid, iodoacetamide or N-ethylmaleimide. Homodimer. Fixes nitrogen very poorly; has very slow diazotrophic growth on nitrogen-free agar plates. A double nifS-nifV deletion no longer fixes nitrogen. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. NifS/IscS subfamily.