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 Zn2+ concentrations, is stimulated by high Zn2+ and Pb2+ 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 σ54-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 Zn2+ and Pb2+ 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 Zn2+ and Pb2+ concentrations and contributes to metal tolerance by activating the expression of zraP. The redesignation of hydH/G as zraS/R is suggested.