Abstract
Hybrid cluster proteins (HCP) contain two types of Fe/S clusters, namely a [4Fe-4S](2+/1+) or [2Fe-2S](2+/1+) cluster and a novel type of hybrid cluster, [4Fe-2S-2O], in the as-isolated state. Although first isolated from anaerobic sulfate-reducing bacteria, the analysis of the genomic sequences reveals that genes encoding putative hybrid cluster proteins are present in a wide range of organisms, aerobic, anaerobic, or facultative, from the Bacteria, Archaea, and Eukarya domains. Despite a detailed spectroscopic and structural characterization, the precise physiological function of these proteins remained unknown. The present work shows that the transcription of the Escherichia coli hcp gene is induced by hydrogen peroxide, and this induction is regulated by the redox-sensitive transcriptional activator, OxyR. The E. coli hcp mutant strain exhibits higher sensitivity to hydrogen peroxide, a behavior that reverts to the wild type phenotype once a plasmid carrying the hcp gene is reintroduced. Furthermore, the purified HCPs from E. coli and Desulfovibrio desulfuricans ATCC 27774 show an alternative enzymatic activity, which under physiological conditions exhibited K(m) values for hydrogen peroxide (approximately 0.3 mM) within the range of other peroxidases. Altogether, the results reveal that HCP is involved in oxidative stress protection.
Highlights
In Escherichia coli Hybrid cluster proteins (HCP) was detected by immunoblotting in cells grown anaerobically with nitrate or nitrite [4]
E. coli hcp Is Induced by Hydrogen Peroxide in an oxyR-dependent Mode—In previous studies, E. coli HCP could be detected only in cells grown under anaerobic conditions using nitrate or nitrite as electron acceptors [4]
We found that hcp was expressed in cells of E. coli grown aerobically or anaerobically in the absence of any electrons acceptors (Fig. 1), with a higher transcriptional level under fermentative conditions
Summary
The purified HCPs from E. coli and Desulfovibrio desulfuricans ATCC 27774 show an alternative enzymatic activity, which under physiological conditions exhibited Km values for hydrogen peroxide (ϳ0.3 mM) within the range of other peroxidases. OCTOBER 27, 2006 VOLUME 281 NUMBER 43 and D. vulgaris [10] was found to be elevated in response to nitrogen oxides such as nitrate, nitrite, or S-nitrosoglutathione It was observed in the microarray profile of Erwinia chrysanthemi that the transcription level of hcp is highly increased upon plant infection [11]. D. desulfuricans HCP was expressed in equivalent amounts in sulfate- or nitrategrown cells [15] These results strongly suggest that HCP has a distinct physiological function, possibly related to other types of stress [16]. We have discovered that E. coli hcp is induced by hydrogen peroxide, is involved in oxidative stress protection, and is regulated by the peroxide regulator OxyR
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