Distinct roles for U-type proteins in iron-sulfur cluster biosynthesis revealed by genetic analysis of the Bacillus subtilis sufCDSUB operon.

The biosynthesis of iron-sulfur (Fe-S) clusters in Bacillus subtilis is mediated by the SUF-like system composed of the sufCDSUB gene products. This system is unique in that it is a chimeric machinery comprising homologues of E. coli SUF components (SufS, SufB, SufC and SufD) and an ISC component (IscU). B. subtilis SufS cysteine desulfurase transfers persulfide sulfur to SufU (the IscU homologue); however, it has remained controversial whether SufU serves as a scaffold for Fe-S cluster assembly, like IscU, or acts as a sulfur shuttle protein, like E. coli SufE. Here we report that reengineering of the isoprenoid biosynthetic pathway in B. subtilis can offset the indispensability of the sufCDSUB operon, allowing the resultant Δsuf mutants to grow without detectable Fe-S proteins. Heterologous bidirectional complementation studies using B. subtilis and E. coli mutants showed that B. subtilis SufSU is interchangeable with E. coli SufSE but not with IscSU. In addition, functional similarity in SufB, SufC and SufD was observed between B. subtilis and E. coli. Our findings thus indicate that B. subtilis SufU is the protein that transfers sulfur from SufS to SufB, and that the SufBCD complex is the site of Fe-S cluster assembly.

Zinc-Ligand Swapping Mediated Complex Formation and Sulfur Transfer between SufS and SufU for Iron-Sulfur Cluster Biogenesis in Bacillus subtilis.

SufU is a zinc-containing protein involved in mobilization of sulfur from SufS for iron-sulfur cluster biogenesis of Bacillus subtilis. Structural basis for the sulfur transfer in SufS-SufU complex was revealed. A zinc-ligand exchange reaction upon SufS-SufU complexation provides a free thiol from Cys41 of SufU as a sulfur acceptor.