Catalyzes the removal of elemental sulfur from cysteine to produce alanine (Probable). Participates in the biosynthesis of metalloclusters by providing the inorganic sulfur required for Fe-S core formation. One acceptor is Whib3, on which this enzyme assembles a 4Fe-4S cluster. It can use both L-cysteine and L-selenocysteine as substrates. [sulfur carrier]-H + L-cysteine = [sulfur carrier]-SH + L-alanine Homodimer. It interacts with Whib3, Acn, SdhB, FdhF and SirA, other S acceptors. Cells lacking this gene are microaerophilic and hypersensitive to oxidative stress. Moreover, they show impaired Fe-S cluster-dependent enzyme activity. IscU is absent from the mycobacterial ISC operon. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. NifS/IscS subfamily.