Master enzyme that delivers sulfur to a number of partners involved in Fe-S cluster assembly, tRNA modification or cofactor biosynthesis. Catalyzes the removal of elemental sulfur and selenium atoms from cysteine and selenocysteine to produce alanine. Functions as a sulfur delivery protein for Fe-S cluster synthesis onto IscU, an Fe-S scaffold assembly protein, as well as other S acceptor proteins. Also functions as a selenium delivery protein in the pathway for the biosynthesis of selenophosphate. [sulfur carrier]-H + L-cysteine = [sulfur carrier]-SH + L-alanine Cofactor biosynthesis; iron-sulfur cluster biosynthesis. Homodimer. Forms a heterotetramer with IscU, interacts with other sulfur acceptors. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. NifS/IscS subfamily.