In at least single eukaryote, Tetrahymena, the pre-rRNA molecule holds an intron.  Erasing  of  the  intron  in running  processing  of  the  pre-rRNA  does  not require  the  assistance  of  any  protein!  Instead,  in  the  presence  of  guanosine, GDP, GMP   or  GTP,  the  intron  excises  itself is a  phenomenon  known  as  self- splicing.  This was the first expression of ribozymes, which is, catalytic RNA molecules which catalyze particular reactions. The Self-splicing introns have also been discovered  in  some  eukaryotic  mRNAs  and  even  peptidyl  transferase  a  main activity in protein synthesis is now known to be a ribozyme.