Thermograms of whole cells of obtained by differential scanning calorimetry contained ten main peaks (denoted and ) occurring at temperatures of approximately 25, 54, 61, 71, 76, 81, 95, 105, 118 and 124 °C, respectively. After cooling to 5 °C and reheating, peaks denoted and were observed at 23, 73 and 94 °C, respectively. By examining thermograms of different cell fractions we have identified the following thermal denaturation events. During primary heating there is a broad endotherm ()beginning below 20 °C and extending to just above 40 °C that is caused by melting of membrane lipids. Superimposed on this is an exothermic process associated with a change of state of the peptidoglycan. The first irreversible denaturation event occurs just above 47 °C, associated with the onset of denaturation of the 30S ribosomal subunit and soluble cytoplasmic proteins. Ribosome melting is a complex process occurring between 47 and 85 °C and is characterized by peaks and . Peak at 75-76 °C is of unknown identity but may possibly represent melting of tRNA. Peak at 95 °C results from melting of a portion of the cellular DNA combined with denaturation of a cell wall component. Peak at 105 °C is multicomponent and may be caused by melting of a different region of DNA together with denaturation of another cell wall component. The complex events denoted and at 118 and 125 °C, respectively, are associated with denaturation of a component of the cell envelope, and possibly also of DNA. Following cooling and reheating there is a broad endotherm with a maximum at 23 °C caused by remelting of membrane lipid and a very broad endotherm extending between 40 and 100 °C caused by the remelting of ribosomal RNA. Peak at 94 °C is caused by the melting of reannealed DNA. Additional features not appearing in whole cells were evident in some cell fractions. These observations should allow us to distinguish events that may lead to loss of viability from those that do not.


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