The hydH/G Genes from Escherichia coli code for a zinc and lead responsive two-component regulatory system.
The hydH/G genes from Escherichia coli code for a two-component regulatory system that has been implicated in the regulation of hydrogenase 3 formation. In a detailed study of the function of HydH/G employing hycA'-'lacZ reporter gene fusions, it was shown that HydH/G indeed led to a stimulation of activation of the hycA promoter responsible for hydrogenase 3 synthesis but only when hydG is overexpressed from a plasmid in a strain lacking FhlA. Since the stimulation was not observed with an fdhF'-'lacZ fusion, and since it was independent from a functional hydH gene product, it must be considered as unspecific cross-talk. An extensive search for the actual physiological signal of HydH/G showed that the system responds to high concentrations of zinc or lead in the medium. Expression of zraP, a gene inversely oriented to hydH/G whose product seems to be involved in acquisition of tolerance to high Zn(2+) concentrations, is stimulated by high Zn(2+) and Pb(2+) concentrations and this stimulation requires both HydH and HydG. Purified HydG in the presence of phosphoryl donors binds to a region within the zraP-hydHG intergenic region that is characterised by two inverted repeats separated by a 14 bp spacer. Putative -12/-24 sigma(54)-dependent promoter motifs are present upstream of both the zraP and the hydHG transcriptional units; in accordance, transcription of zraP is strictly dependent on the presence of a functional rpoN gene. The expression of hydH/G is autoregulated: high Zn(2+) and Pb(2+) concentrations lead to a significant increase of the HydG protein content which took place only in a hydH(+) genetic background. Since HydH binds to membranes tightly, it is assumed that the HydH/G system senses high periplasmic Zn(2+) and Pb(2+) concentrations and contributes to metal tolerance by activating the expression of zraP. The redesignation of hydH/G as zraS/R is suggested.
Lehrstuhl für Mikrobiologie der Universität München, Maria-Ward-Str. 1a, D-80638 Munich, Germany.
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Leonhartsberger
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Huber
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Lottspeich
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Böck
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D001483:Base Sequence; D003001:Cloning, Molecular; D018983:DNA Footprinting; D004269:DNA, Bacterial / Q000032:analysis / Q000378:metabolism; D004268:DNA-Binding Proteins; D012321:DNA-Directed RNA Polymerases / Q000502:physiology; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins; D005560:Formate Dehydrogenases / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D006864:Hydrogenase / Q000235:genetics; D007854:Lead / Q000378:metabolism*; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000235:genetics; D051877:RNA Polymerase Sigma 54; D012689:Sequence Homology, Nucleic Acid; D012808:Sigma Factor / Q000502:physiology; D015534:Trans-Activators / Q000235:genetics* / Q000502:physiology; D014158:Transcription, Genetic; D015032:Zinc / Q000378:metabolism*
D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; D009097:Multienzyme Complexes; D012808:Sigma Factor; D015534:Trans-Activators; C497751:rpoN protein, E coli; C114106:zraP protein, E coli; C423908:zraR protein, E coli; C423907:zraS protein, E coli; D007854:Lead; D006864:Hydrogenase; D005560:Formate Dehydrogenases; C054017:formate hydrogenlyase; D012321:DNA-Directed RNA Polymerases; D051877:RNA Polymerase Sigma 54; D015032:Zinc