Roles and specific functions of homologous enzymes involved in biogenesis of active-form sulfur and active-form selenium

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Mechanism and physiological role of cysteine desulfurase which decomposes cysteine into alanine and sulfur and its homolog, selenocysteine lyase, were studied. Two cysteine desulfurases from a cyanobacterium were used to generate an iron-sulfur cluster in an iron-sulfur protein. The enzymes efficiently converted apo-ferredoxin into [2Fe-2S] ferredoxin by using L-cysteine as a sulfur source. Selenocysteine lyase from Escherichia coli was employed to synthesize selenophosphate in vitro. Selenophosphate was efficiently produced in the presence of selenophosphate synthetase, selenocysteine, and ATP.Three gene-disrupted E.coli strains, in which a gene for each cysteine desulfurase homolog was disrupted, were constructed to investigate role of each enzyme. Growth rate of the iscS-disruptant was significantly decreased on a LB plate medium. The iscS-disruptant was transformed with plasmids containing genes for CSD, CsdB, IscS and their mutant enzymes, and growth-rates of the transformatnts were investigated on various solid media. Introduction of csdB into the iscS-disruptant recover the growth-rate of the strain to the level as the wild-type strain. The result suggests that the over-produced CsdB can be a back-up of IscS in vivo. On the other hand, expression of CSD did not complement the growth of the iscS-disruptant. Thus, the in vivo functions of CSD and CsdB are different from each other, although they show about 50% amino acid identity.