Role of NifS homologs in selenium metabolism

Abstract

Three NifS-like proteins, IscS, CSD, and CsdB, from Escherichia coli catalyze the removal of sulfur and selenium from L-cysteine and L-selenocysteine, respectively, to form L-alanine. These enzymes are proposed to deliver a sulfur atom for iron-sulfur cluster, thiamin, 4-thiouridine, biotin, and molybdopterin. It was reported that selenium mobilized from free selenocysteine is incorporated specifically into a selenoprotein and tRNA in vivo, supporting the involvement of the NifS-like proteins in selenium metabolism. We here report evidence that a strain lacking IscS is incapable of synthesizing mnm⁵se²U and its precursor mnm⁵s²U in tRNA, suggesting that the sulfur atom released from L-cysteine by the action of IscS is incorporated into mnm⁵s²U. The lack of IscS also caused a significant loss of the selenium-containing formate dehydrogenase H. Together, these results suggest a dual function of IscS in sulfur and selenium metabolism.