Electron probe X-ray microanalysis of bacterial DNA

X-ray microanalysis provides a potentially useful technique for the detection, quantitation and statistical analysis of cation levels in microsamples of freshly-isolated bacterial DNA. The dilution of the sample deposited on the electron microscope grid is important both for the visualisation of discrete macromolecules and also for the microanalytical detection of a full range of divalent cations. Approximately 50 detectable cations (30 monovalent, 20 divalent) were present per 100 phosphorus atoms, which is equivalent to one detectable cation per 2 nucleotides. Significant correlations in the occurrence of K, Ca, Ni and Zn with continuum and also with phosphorus suggests that these cations are distributed homogeneously in the sample, while Mn, Fe and Cu showed no such correlations. The occurrence of K in the different spectral analyses did not show any significant correlation with any divalent cations, whereas significant correlations did occur between divalent cations. This suggest that fundamental differences occur between monovalent and divalent cations in their association with the DNA molecule.