De-embedded ultrathin sections of ethanol-fixed Tipula Iridescent Virus particles were incubated with pepsin at pH 1.8, trypsin at pH 7.7, and DNase at pH 7.7. The outer shell of the particles, but not an inner core, was removed by the action of pepsin. Conversely, the inner core, but not the outer shell, was removed by the action of trypsin and DNase in combination, but not by either enzyme acting alone. These results are taken to mean that the outer shell of the particles is protein in nature and the inner core is nucleoprotein. Whole virus particles were also exposed to the same 3 enzymes. Trypsin and/or DNase had no effect on the whole particles, while pepsin at pH 1.8 digested away the outer shell of the particles and released an intact core, resistant to pepsin. The protein nature of the digested outer shells and the nucleoprotein nature of the released cores were confirmed by ultraviolet absorption spectra. Chemical analyses showed that the cores contain 89 per cent of the whole virus phosphorus but only 35 per cent of the nitrogen, while the outer shells contain only 5 per cent of the phosphorus but 63 per cent of the nitrogen. On the basis of nitrogen: phosphorus ratios the composition of the cores is estimated to be about 30 per cent DNA and 60 to 65 per cent protein.

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