<p>Prokaryotes are continually exposed to variations in their environment. Survival in unstable milieu requires a wide range of transcriptional regulators (TRs) that respond to specific environmental and cellular signals by modulating gene expression and provide an appropriate physiological response to external stimuli. These adaptive responses to environmental signals are mostly mediated by TRs from one of two families: the single or the two component signal transduction systems (1CSTS; 2CSTS). In this thesis the structural analysis of two 1CSTS – DntR and NikR − are presented. One study was carried out to try to develop a bacterial biosensor for synthetic dinitrotulenes compounds, the other to characterise the Ni-sensing mechanism that contributes to the acid adaptation of the human pathogen<i> Helicobacter pylori.</i> DntR belongs to the LysR family and the crystal structures obtained have allowed the proposal a model of the interaction of DntR with salicylate inducer as well as giving insights into the signal propagation mechanism in LysR-type transcription factors (<b>paper I</b>). DntR mutant crystal structures combined with the modelling of DntR-2,4-dnt interactions led to the design of a DntR mutant that has a limited response to 2,4-dnt in a whole cell biosensor system (<b>paper 2</b>). Crystal structures of apo-NikR from <i>H. pylori </i>(HpNikR) and of Ni-bound intermediary states of the protein were obtained. The latter have helped in unravelling the Ni incorporation and selectivity mechanisms of NikRs and have shown a strong cooperativity between conformational changes in the Ni binding domain with movements of the DNA binding domain (<b>paper 3</b>). Biochemical studies and comparisons of the HpNikR crystal structures with those of NikR homologues strongly suggest that HpNikR has evolved different surface properties (<b>paper 4</b>) and a new mode of DNA binding. </p>

Prokaryotes are continually exposed to variations in their environment. Survival in unstable milieu requires a wide range of transcriptional regulators (TRs) that respond to specific environmental and cellular signals by modulating gene expression and provide an appropriate physiological response to external stimuli. These adaptive responses to environmental signals are mostly mediated by TRs from one of two families: the single or the two component signal transduction systems (1CSTS; 2CSTS). In this thesis the structural analysis of two 1CSTS – DntR and NikR − are presented. One study was carried out to try to develop a bacterial biosensor for synthetic dinitrotulenes compounds, the other to characterise the Ni-sensing mechanism that contributes to the acid adaptation of the human pathogen Helicobacter pylori. DntR belongs to the LysR family and the crystal structures obtained have allowed the proposal a model of the interaction of DntR with salicylate inducer as well as giving insights into the signal propagation mechanism in LysR-type transcription factors (<b>paper I</b>). DntR mutant crystal structures combined with the modelling of DntR-2,4-dnt interactions led to the design of a DntR mutant that has a limited response to 2,4-dnt in a whole cell biosensor system (<b>paper 2</b>). Crystal structures of apo-NikR from H. pylori (HpNikR) and of Ni-bound intermediary states of the protein were obtained. The latter have helped in unravelling the Ni incorporation and selectivity mechanisms of NikRs and have shown a strong cooperativity between conformational changes in the Ni binding domain with movements of the DNA binding domain (<b>paper 3</b>). Biochemical studies and comparisons of the HpNikR crystal structures with those of NikR homologues strongly suggest that HpNikR has evolved different surface properties (<b>paper 4</b>) and a new mode of DNA binding.