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Age-dependent lability of chloroplast ribosomal RNA from Nicotiana rustica When rRNA of tobacco chloroplasts ( Nicotiana rustica ) is heated for 1 min, the 1.1 · 10 6 (23 S) RNA is fragmented, whereas the 0.56 · 10 6 (16 S) RNA remains unaltered. In the presence of Mg 2+ , the dissociation begins at 45–50°C. At 60°C, the 1.1 · 10 6 RNA is broken down almost quantitatively into fragments of specific molecular weights. This thermal dissociation is reduced in the presence of Mg 2+ and in buffer of high ionic strength. When chloroplast ribosomes are heated, the RNA remains intact. The breaks in the RNA molecules are neither artefacts of the extraction procedure, nor produced during the heating by nucleases that resisted phenol extraction. A similar conversion of 1.1 · 10 6 RNA is brought about by 80% dimethylsulfoxide or 6 M urea at 0°C, i.e. by all treatments separating the base-pairing responsible for the secondary structure of the RNA. The thermal dissociation depends on the age of the RNA. Ribosomal RNA precursors of chloroplasts (p23 and p16) are resistant to heat treatment, as well as all newly synthesized mature rRNAs (revealed by pulse labelling). However, in rooted leaves the radioactive 1.1 · 10 6 rRNA, labelled by a 32 P i pulse, has the characteristic breaks in the molecule after 5 days and is fragmented during heating in the same way as all accumulated 1.1 · 10 6 RNA. The thermal instability of the 1.1 · 10 6 rRNA presumably is the consequence of “hidden breaks” in the polynucleotide chain originating by nuclease action in the course of ageing of the chloroplast ribosomes. |
