The two-component system of a novel copper resistant operon of Marinobacter hydrocarbonoclasticus

Dissertation for the Master’s Degree in Structural and Functional Biochemistry

The majority of bacterial heavy metal resistance systems are regulated by twocomponent signal transduction systems. Stimuli from the environment interact with the histidine kinase, which in turn activates the response regulator by phosphorylation. The effector domain of the response regulator then binds to DNA, eliciting the specific response. Analysis of the Marinobacter hydrocarbonoclasticus genome revealed the presence of genes, copXAB, that code for proteins associated with copper response. The biochemical characterization of the two-component signal transduction system, copSR, is of interest due to the vital role it plays in the regulation of expression of the copXAB operon. The genes that encode for the CopR and CopS_C (cytosolic sensor domain of CopS) proteins were heterologously expressed in E.coli and expression was optimized for the production of soluble protein using LB medium. Due to solubility problems, the genes that code for these proteins were cloned as hexahistidine or glutathione S-transferase fusion proteins. CopR and its domains were optimally expressed at 16°C for 16 and 3 h after induction, respectively, whilst CopS_C was expressed at 37°C during 3 h after induction. Proteins were purified using different chromatographic strategies, most of them using affinity chromatography. The yields of pure protein per liter of growth culture obtained after complete purification from the soluble cellular extract were: 0.14 to 0.23 mg/L for CopR; 0.42 mg/L, CopR_NHis6; for the CopR_CHis6 it was 0.16 mg/L and 4.2 mg/L of CopS_C. The molecular mass of each protein was determined by gel filtration, 31 kDa for CopR, 17.5 kDa for CopR_NHis6, 15.1 kDa for CopR_CHis6 and 38.2 kDa for CopS_C. In the case of CopS_C there is the possibility that a dimer is formed, which should be evaluated. From the evaluation of disulfide bonds, using SDS PAGE and PAGE gels, all proteins or protein domains appeared to be monomers when in the presence of β-MEtOH. Circular dichroism evaluated the state of folding of the CopS_C and CopR proteins, which were shown to be folded in which the α-helix structures predominate. A model structure for CopR was also determined which agrees with this analysis. However, in the case of the CopR domains, the data obtained merely indicate folding, due to the low concentrations of the proteins. Phosphorylation and electrophoresis mobility shift assays of the CopR protein were, for the most part, inconclusive. However, in the absence of BSA, formation of the CopR:DNA complex in a gel filtration column is observed, though requires additional evaluation.