Accessibility of phosphodiester bonds in the yeast ribosomal 5 S RNA protein complex

The tertiary structure of the protein-associated yeast ribosomal 5 S RNA was examined using ethylnitrosourea reactivity as a probe for phosphodiester bonds. A reduced reactivity was consistently observed in at least nine residues within four distinct regions of the RNA sequence. Seven of these were also observed in three regions of the free RNA molecule while two, A27 and G30, were only present in the ribonucleoprotein complex. The results strongly suggest that the tertiary structure of the free eukaryotic 5 S RNA is largely conserved in the 5 S RNA-protein complex although it appears to be further stabilized in interaction with the ribosomal protein.