Modified sequences in yeast ribosomal RNA.

26S and 17S yeast ribosomal RNA were digested with T 1 plus pancreatic ribonuclease and the products were fractionated by two-dimensional electrophoresis. Besides the expected standard products (type (Ap) n Np, where N is C, U or G) several non-standard products were found to be present in the digests. The latter products include methylated oligonucleotides and pseudouridylic acid (ψp)-containing fragments. The primary structure and molar frequency of these modified products were determined. They appeared to be present in approximately integral molar amounts. Several of these oligonucleotides contain more than one modification. The total number of ψp-residues in 26S and 17S yeast rRNA was estimated to be about 32 and 14, respectively.