Studies on the secondary structure of nuclear ribonucleic acids.

Rat liver nuclei were separated into two fractions, the nucleolus and the nucleoplasm, by a combined salt-enzymic extraction. RNA was purified from both sources and analysed on sucrose density gradients. In both fractions a prominent heterogeneous RNA class with mean sedimentation coefficient of 18S was found. This material was analysed by measuring the rate of reaction with formaldehyde, the ultraviolet absorbance-temperature profile, the spectrophotometric observation of conformational changes as a function of pH, the spectrophotometric titration of uracil and guanine residues and the effect of both temperature and ionic strength on the spectrophotometric titration of cytosine residues. Nucleoplasmic 18S RNA fraction exhibited, on heating and also by adjustment of the pH to 2.5, a hyperchromicity of about 16%, close to that observed, in control experiments, for ribosomal RNA (22%). Titration of cytosine residues in solutions containing 1mm-NaCl and 0.1m-NaCl yielded pK values equal to 4.41 and 3.84 respectively. These results suggest that this RNA fraction is composed of structurally complex polymers. The hypochromicity of the nucleolar 18S RNA fraction determined by heating or adjusting the pH to 2.5, was not greater than 6% of the initial value. The rate of reaction with formaldehyde was 88% of that observed for the hydrolysed 18S fraction which suggested only 12% hydrogen bonding. pK values for uracil and guanine residues were 10.1 and 10.05 respectively. Titration of cytosine residues yielded a pK of 4.10, which was found to be independent of temperature and ionic strength variations.