Catalyzes the removal of elemental sulfur atoms from cysteine to produce alanine. Able to reassemble a removed [2Fe-2S] cluster of an apo-ferredoxin. Shows a selenocysteine lyase activity approximately 280-fold higher than its cysteine desulfurase activity. [sulfur carrier]-H + L-cysteine = [sulfur carrier]-SH + L-alanine AH2 + L-selenocysteine = A + H(+) + hydrogenselenide + L-alanine DTT increases activity up to 46%, but has an inhibitory effect at concentrations higher than 5 mM in vitro. Optimum pH is 7.5. Remains partially active at pH 11. Optimum temperature is 55-60 degrees Celsius in the presence of 0-0.5 M KCl and 70-75 degrees Celsius in the presence of 2.5-3.0 M KCl. Cofactor biosynthesis; iron-sulfur cluster biosynthesis. Homodimer. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. Csd subfamily. Extended N-terminus.