Shigella dysenteriaeShuS Promotes Utilization of Heme as an Iron Source and Protects against Heme Toxicity

Abstract

Shigella dysenteriae serotype 1, a major cause of bacillary dysentery in humans, can use heme as a source of iron. Genes for the transport of heme into the bacterial cell have been identified, but little is known about proteins that control the fate of the heme molecule after it has entered the cell. The shuS gene is located within the heme transport locus, downstream of the heme receptor gene shuA. ShuS is a heme binding protein, but its role in heme utilization is poorly understood. In this work, we report the construction of a chromosomal shuS mutant. The shuS mutant was defective in utilizing heme as an iron source. At low heme concentrations, the shuS mutant grew slowly and its growth was stimulated by either increasing the heme concentration or by providing extra copies of the heme receptor shuA on a plasmid. At intermediate heme concentrations, the growth of the shuS mutant was moderately impaired, and at high heme concentrations, shuS was required for growth on heme. The shuS mutant did not show increased sensitivity to hydrogen peroxide, even at high heme concentrations. ShuS was also required for optimal utilization of heme under microaerobic and anaerobic conditions. These data are consistent with the model in which ShuS binds heme in a soluble, nontoxic form and potentially transfers the heme from the transport proteins in the membrane to either heme-containing or heme-degrading proteins. ShuS did not appear to store heme for future use.The Shigella spp. are the primary cause of bacillary dysentery in humans. Following ingestion, this pathogen transits the digestive system to the colon, where it invades the cells of the colonic epithelium. It then replicates inside the epithelial cells and spreads to adjacent cells. The resulting inflammation and tissue damage are responsible for the bloody diarrhea characteristic of dysentery.

Structure of the Escherichia coli O157:H7 Heme Oxygenase ChuS in Complex with Heme and Enzymatic Inactivation by Mutation of the Heme Coordinating Residue His-193

“…Despite the evidence that genes homologous to chuS have been shown to be up-regulated under iron-limiting conditions, that these homologues prevented heme toxicity in S. dysenteriae (7), and that no other heme-degrading enzyme has been identified in any E. coli strain, the precise role for ChuS and its homologues has been downplayed in recent reports to be that of a cytoplasmic trafficker of heme or a nonspecific oligomeric complex that binds to DNA (15,25). By carefully examining the spectral progression and CO measurement of ChuS-catalyzed degradation of heme in the presence of minimal amounts of ascorbic acid, we conclude that ChuS specifically degrades heme and suggest that His-193 is the essential amino acid responsible, similar to His-25 in HO-1 (11).…”

“…A Shigella dysenteriae chromosomal knock-out of shuS, a homologue to chuS, was shown to be defective in utilizing heme as an iron source, and expression of ShuS was up-regulated when challenged with iron-limiting conditions (7). Furthermore, shuS was implicated to have a cytoprotective role against heme toxicity in that growth of the shuS knock-out mutant was impaired at intermediate concentrations of heme (15 M) and that shuS was absolutely required for colony growth at high heme concentrations (40 M) (7).…”

Non-heme Induction of Heme Oxygenase-1 Does Not Alter Cellular Iron Metabolism

“…Ischemia-reperfusion injury will also bring about significant intravascular hemolysis (31,32), again supplying substrate for HO. In these cases, the detoxifying effects of heme oxygenase include the clearance of heme, which in itself is cytotoxic (33)(34)(35). In contrast, it has been shown that heme oxygenase overexpression or induction can confer cytoprotection in the absence of aberrant, extracorpuscular hemoproteins (5, 13, 36 -38).…”

In vivo expression technology identifies a type VI secretion system locus in Burkholderia pseudomallei that is induced upon invasion of macrophages

“…The BhuU and BhuV orthologues, HemU/HmuU and HemV/HmuV, form the permease and ATPase components, respectively, of the cytoplasmic membrane haem transporter. The Shigella BhuS orthologue, ShuS, has been suggested to act as a shuttle protein that potentiates the utilization or degradation of haem, depending upon the prevailing conditions (Wyckoff et al, 2005). Upstream of bhuR is the sequence 59-TTGAATN 17 TACAAT-39, which is similar to the consensus promoter sequence recognized by the major form of RNA polymerase.…”