Expression, Purification, and Characterization of Enzyme IIAglc of the Phosphoenolpyruvate:Sugar Phosphotransferase System of Mycoplasma capricolum

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Abstract
The gene encoding enzyme IIA(glc) (EIIA) of the phosphoenolpyruvate:sugar phosphotransferase system of Mycoplasma capricolum was cloned into a regulated expression vector. The purified protein product of the overexpressed gene was characterized as an active phosphoacceptor from HPr with a higher pI than previously described EIIAs. M. capricolum EIIA was unreactive with antibodies directed against the corresponding proteins from either Gram-positive or Gram-negative bacteria. Enzyme IIA(glc) behaved as a homogeneous, monomeric species of 16,700 Mr in analytical ultracentrifugation. The circular dichroism far-UV spectrum of EIIA reflects a low alpha-helical content and predominantly beta-sheet structural content: temperature-induced changes in ellipticity at 205 nm showed that the protein undergoes reversible, two-state thermal unfolding with Tm = 70.0 +/- 0.3 degrees C and a van't Hoff deltaH of 90 kcal/mol. Enzyme I (64,600 Mr) from M. capricolum exhibited a monomer-dimer-tetramer association at 4 and 20 degrees C with dimerization constants of log K(A) = 5.6 and 5.1 [M(-1)], respectively, in sedimentation equilibrium experiments. A new vector, capable of introducing an N-terminal His tag on a protein, was developed in order to generate highly purified heat-stable protein (HPr). No significant interaction of EIIA with HPr was detected by gel-filtration chromatography, intrinsic tryptophanyl residue fluorescence changes, titration calorimetry, biomolecular interaction, or sedimentation equilibrium studies. While Escherichia coli EIIA inhibits Gram-negative glycerol kinase activity, the M. capricolum EIIA has no effect on the homologous glycerol kinase. The probable regulator of sugar transport systems, HPr(Ser) kinase, was demonstrated in extracts of M. capricolum and Mycoplasma genitalium. Gene mapping studies demonstrated that, in contrast to the clustered arrangement of genes encoding HPr and enzyme I in E. coli, these genes are located diametrically opposite in the M. capricolum chromosome.