SufU Is an Essential Iron-Sulfur Cluster Scaffold Protein in Bacillus subtilis

Abstract

ABSTRACT Bacteria use three distinct systems for iron-sulfur (Fe/S) cluster biogenesis: the ISC, SUF, and NIF machineries. The ISC and SUF systems are widely distributed, and many bacteria possess both of them. In Escherichia coli , ISC is the major and constitutive system, whereas SUF is induced under iron starvation and/or oxidative stress. Genomic analysis of the Fe/S cluster biosynthesis genes in Bacillus subtilis suggests that this bacterium's genome encodes only a SUF system consisting of a sufCDSUB gene cluster and a distant sufA gene. Mutant analysis of the putative Fe/S scaffold genes sufU and sufA revealed that sufU is essential for growth under minimal standard conditions, but not sufA . The drastic growth retardation of a conditional mutant depleted of SufU was coupled with a severe reduction of aconitase and succinate dehydrogenase activities in total-cell lysates, suggesting a crucial function of SufU in Fe/S protein biogenesis. Recombinant SufU was devoid of Fe/S clusters after aerobic purification. Upon in vitro reconstitution, SufU bound an Fe/S cluster with up to ∼1.5 Fe and S per monomer. The assembled Fe/S cluster could be transferred from SufU to the apo form of isopropylmalate isomerase Leu1, rapidly forming catalytically active [4Fe-4S]-containing holo -enzyme. In contrast to native SufU, its D43A variant carried a Fe/S cluster after aerobic purification, indicating that the cluster is stabilized by this mutation. Further, we show that apo -SufU is an activator of the cysteine desulfurase SufS by enhancing its activity about 40-fold in vitro . SufS-dependent formation of holo -SufU suggests that SufU functions as an Fe/S cluster scaffold protein tightly cooperating with the SufS cysteine desulfurase.