Rat liver ribosomes were incubated in 4 M urea at 37°. Autodegradation of the ribosomal RNA (rRNA) was optimum at pH 8.0. Ribonucleolytic activity did not decrease on repeated washing of the ribosomes but disappeared completely on heating them at 100° for 5 min. After 3 h incubation about 40-45 % of the total rRNA was soluble in 5 % perchloric acid. The degradation products were separated by DEAE-cellulose column chromatography and 3′- (or 2′-), 5′-terminal hydroxy phosphorylated dinucleotide (pNpNp) was isolated and identified with other nucleosides and 3′-oligonucleotides. pNpNp constituted 4-5 % of the total rRNA. Escherichia coli rRNA was also hydrolyzed in the autodegradation system of rat liver ribosomes and 50-60 % became soluble in 5 % perchloric acid. In this case also, pNpNp was isolated and identified. The base composition of pNpNp in the degradation products of rat liver rRNA was pUpGp 32.7 %, pUpCp 17.8 %, pUpAp 26.2 % and pUpUp 23.4 %.
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