[Nuclear ribonucleoproteins containing messenger RNA. 12. Fluorescence studies of the secondary structure of pre-mRNA in nuclear RNP-particles].

Secondary structure of pre-mRNA in nuclear ribonucleoprotein particles (30S-particles) was examined using fluorescent dyes: acridine orange, acriflavine and ethidium bromide. Comparison of ethidium bromide and acriflavine adsorption isotherms for RNP-particles and free RNA and a study of acridine orange dimerization on binding to RNP revealed that 70% of pre-mRNA in 30S-particles is accessible for the dye binding. Dye molecules were adsorbed on double-stranded sequences (11--12% of the total amount of RNA in 30S-particles) and on the single-stranded parts of RNA (58--59% of 30S-particles), the rest part of RNA was unaccessible for the dye binding. A method involving measurements of acriflavine fluorescence quantum yields was used for the determination of nucleotide composition of double-stranded parts of RNA in the 30S-particles. AU-nucleotide content thus obtained was approximately 50%, as was established also for free pre-mRNA. Na+ ions weaken the interaction between the protein and pre-mRNA in 30S particles and increase mobility of double-stranded parts of this nucleic acid.