Determination of molecular weight and molecular structure of rat-liver pyruvate carboxylase.

The molecular weight of pyruvate carboxylase isolated from pigeon and rat liver mitochondria was examined using analytical ultracentrifugation and electron microscopy. The enzyme molecule appeared as a tetramer with the four subunits arranged at the corners of a square. Sedimentation studies in the analytical ultracentrifuge, extrapolated to infinite dilution, showed the tetramer to have a molecular weight Mc=0r of 280 000 and an So20,w of 12.7 S. The tetramer could be dissociated into trimers and dimers of lower specific enzymic activity by storage at 4 degrees C or incubation at -- 20 degrees C at low protein concentrations. The isolated trimers and dimers had a molecular weight Mc=0r of 210 000 and 140 000, respectively, and an So20,w of 10.85 S and 7.55 S, respectively. Incubation with 2 M urea at 20 degrees C yielded enzymically inactive subunits (Mc=0r = 70 000; So20,w = 4.95 S). The molecular weights (for pyruvate carboxylase and its subunits), as calculated from the subunit diameter observed in the electron microscope, were consistent with the values obtained from sedimentation studies.     

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Mutation rate dynamics reflect ecological change in an emerging zoonotic pathogen

Mutation rates vary both within and between bacterial species, and understanding what drives this variation is essential for understanding the evolutionary dynamics of bacterial populations. In this study, we investigate two factors that are predicted to influence the mutation rate: ecology and genome size. We conducted mutation accumulation experiments on eight strains of the emerging zoonotic pathogen Streptococcus suis. Natural variation within this species allows us to compare tonsil carriage and invasive disease isolates, from both more and less pathogenic populations, with a wide range of genome sizes. We find that invasive disease isolates have repeatedly evolved mutation rates that are higher than those of closely related carriage isolates, regardless of variation in genome size. Independent of this variation in overall rate, we also observe a stronger bias towards G/C to A/T mutations in isolates from more pathogenic populations, whose genomes tend to be smaller and more AT-rich. Our results suggest that ecology is a stronger correlate of mutation rate than genome size over these timescales, and that transitions to invasive disease are consistently accompanied by rapid increases in mutation rate. These results shed light on the impact that ecology can have on the adaptive potential of bacterial pathogens.