Bacterial Approaches for Assembling Iron-Sulfur Proteins

Abstract

Building iron-sulfur (Fe-S) clusters and assembling Fe-S proteins are essential actions for life on Earth. The three processes that sustain life, photosynthesis, nitrogen fixation, and respiration, require Fe-S proteins. Genes coding for Fe-S proteins can be found in nearly every sequenced genome. Fe-S proteins have a wide variety of functions, and therefore, defective assembly of Fe-S proteins results in cell death or global metabolic defects. Compared to alternative essential cellular processes, there is less known about Fe-S cluster synthesis and Fe-S protein maturation. Moreover, new factors involved in Fe-S protein assembly continue to be discovered. These facts highlight the growing need to develop a deeper biological understanding of Fe-S cluster synthesis, holo-protein maturation, and Fe-S cluster repair. Here, we outline bacterial strategies used to assemble Fe-S proteins and the genetic regulation of these processes. We focus on recent and relevant findings and discuss future directions, including the proposal of using Fe-S protein assembly as an antipathogen target.

Keywords: ISC; NIF; SUF; bacteria; genetic regulation; iron; iron regulation; iron utilization; iron-sulfur cluster; metalloproteins; metalloregulation; sulfide; sulfur.

FIG 1

General mechanism of bacterial Fe-S protein assembly. Monoatomic Fe²⁺ and S⁰ are combined with electrons on a proteinaceous molecular scaffold forming an Fe-S cluster. The Fe-S cluster is transferred to one or more carrier proteins before being transferred to an apo-protein forming a holo-protein. Reactive oxygen species (ROS) can either damage the Fe-S cluster, which can subsequently be repaired, or destroy it, resulting in apo-protein formation.

FIG 2

Iron-sulfur cluster synthesis. (A) Structure of IscU from Thermus thermophilus (PDB accession number 2QQ4). The gray ball is a Zn(II) ion, and the side chains of the three ligating cysteines are highlighted. (B) Structure of SufBC₂D from Escherichia coli (PDB accession number 5AWF). SufC is shown in green, and SufB and SufD are shown in purple and tan, respectively. (C) Working models for ISC- and SUF-directed iron-sulfur protein maturation in Escherichia coli.

FIG 3

Iron-sulfur cluster carriage. (A) Structure of the A-type carrier IscA from Thermosynechococcus elongatus (PDB accession number 1X0G) with a [2Fe-2S] cluster bound. (B) Structure of Nfu from Staphylococcus epidermidis (PDB accession number 1XHJ). The cysteine thiols that are proposed iron-sulfur cluster ligands are highlighted. (C) Schematic representation of iron-sulfur cluster scaffolds and carriers.

FIG 4

Regulation of iron-sulfur cluster synthesis in Escherichia coli. (A) X-ray structure of an apo-IscR monomer with the proposed Fe-S cluster ligands (C92A, C98A, C104A, and H107) highlighted in red (PDB accession number 4HF1). Note that in this IscR variant, the ligating cysteines have been changed to alanines. (B) X-ray structure of dimeric apo-IscR bound to the hya promoter, which is a type 2 binding site (PDB accession number 4HF1). Each monomer is differently colored (blue and pink). (C) Model for the regulation of ISC and SUF expression in Escherichia coli.