SUMMARY: The correlation between the melting temperature of intracellular DNA, determined by differential scanning calorimetry (DSC) of whole bacteria, and its guanine + cytosine (G + C) content, was examined for 58 species of bacteria. Samples of vegetative cells were heated in a Perkin-Elmer DSC-2C at 10 °C min from 5 to 130 °C, cooled to 5 °C and then re-heated as before. Literature values for the mole fraction of G + C, , were linearly related to the temperature, , at which the reversible peak, , observed on the second heating run was at a maximum, via the equation = ( − 73.8)/41.0. This equation accounted for 91.9% of the variance in with 95% confidence limits of ±7.3%, approximately 1.6 times the corresponding uncertainty (±4.5%) quoted by De Ley ( , 738-754, 1970) for estimates based on the spectroscopically determined melting temperature of purified DNA. Random errors of measurement of did not greatly limit the precision of the prediction and it was concluded that factors additional to base composition affected the temperature of DNA melting within the bacterial cell. Displacement of values from the fitted line was particularly noticeable in and species and part of the residual variation appeared to be species specific, possibly caused by differences in intracellular solute concentration.


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