Iron-sulfur (Fe-S) clusters are necessary for the proper functioning of numerous metalloproteins. Fe-S cluster (Isc) and sulfur utilization factor (Suf) pathways are the key biosynthetic routes responsible for generating these Fe-S cluster prosthetic groups in Escherichia coli. Although Isc dominates under normal conditions, Suf takes over during periods of iron depletion and oxidative stress. Sulfur acquisition via these systems relies on the ability to remove sulfur from free cysteine using a cysteine desulfurase mechanism. In the Suf pathway, the dimeric SufS protein uses the cofactor pyridoxal 5-phosphate (PLP) to abstract sulfur from free cysteine, resulting in the production of alanine and persulfide. Despite much progress, the stepwise mechanism by which this PLP-dependent enzyme operates remains unclear. Here, using rapid-mixing kinetics in conjunction with X-ray crystallography, we analyzed the pre-steadystate kinetics of this process while assigning early intermediates of the mechanism. We employed H123A and C364A SufS variants to trap Cys-aldimine and Cys-ketimine intermediates of the cysteine desulfurase reaction, enabling direct observations of these intermediates and associated conformational changes of the SufS active site. Of note, we propose that Cys-364 is essential for positioning the Cys-aldimine for C␣ deprotonation, His-123 acts to protonate the Ala-enamine intermediate, and Arg-56 facilitates catalysis by hydrogen bonding with the sulfhydryl of Cys-aldimine. Our results, along with previous SufS structural findings, suggest a detailed model of the SufS-catalyzed reaction from Cys binding to C-S bond cleavage and indicate that Arg-56, His-123, and Cys-364 are critical SufS residues in this C-S bond cleavage pathway.

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The nickel-responsive transcription factor YqjI represses its own transcription and transcription of the divergent yqjH gene, which encodes a novel ferric siderophore reductase. The intergenic region between the two promoters is complex, with multiple sequence features that may impact YqjI-dependent regulation of its two target promoters. We utilized mutagenesis and DNase I footprinting to characterize YqjI regulation of the yqjH-yqjI intergenic region. The results show that YqjI binding results in an extended footprint at the yqjI promoter (site II) compared to the yqjH promoter (site I). Mutagenesis of in vivo gene reporter constructs revealed that the two YqjI binding sites, while separated by nearly 200 bp, appear to communicate in order to provide full YqjI-dependent regulation at the two target promoters. Thus, YqjI binding at both promoters is required for full repression of either promoter, suggesting that the two YqjI binding sites cooperate to control transcription from the divergent promoters. Furthermore, internal deletions that shorten the total length of the intergenic region disrupt the ability of YqjI to regulate the yqjH promoter. Finally, mutagenesis of the repetitive extragenic palindromic (REP) elements within the yqjH-yqjI intergenic region shows that these sequences are not required for YqjI regulation. These studies provide a complex picture of novel YqjI transcriptional regulation within the yqjH-yqjI intergenic region and suggest a possible model for communication between the YqjI binding sites at each target promoter. Metal ions play an important role in all organisms. It is estimated that about one-third of all proteins require a metal for proper function, with approximately half of (40%) all enzymes having metal bound to them as a cofactor (1). However, even essential transition metals can catalyze spurious side reactions if present in excess. Due to their dual nature, acquisition and intracellular trafficking of essential first-row transition metals, such as iron, zinc, copper, and nickel, are carefully controlled by metal homeostasis systems. Metal homeostasis systems consist of metal transporters, metal storage proteins, and metallochaperones that work in concert to maintain the proper cellular metal quota (2). Expression of metal homeostasis genes is often controlled by specialized transcription factors known as metalloregulatory proteins (3). Besides their functional DNA binding domains, metalloregulatory proteins often have an allosteric metal binding domain that is able to directly sense cellular metal changes (4). Binding or dissociation of the metal ion(s) at the metal binding domain switches the DNA binding domain to increase or decrease its affinity for target promoter(s). This metal-responsive allosteric switch coordinates expression of the genes encoding metal utilizing proteins and other metal homeostasis proteins with cellular metal levels.In a previous study, we discovered that the predicted transcription factor YqjI from Escherichia coli is a nickel-responsive transcr...

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