Ribonuclease catalytic properties

Self-splicing KNA. The precursor to the 26S rRNA of Tetrahymena contains a 413-nucleotide intron, which was shown by Cedi and coworkers to be selfsplicing, i.e., not to require a protein catalyst for maturation.581 582 This pre-rRNA is a ribozyme with true catalytic properties (Chapter 12). It folds into a complex three-dimensional structure which provides a binding site for free guanosine whose 3-OH attacks the phosphorus at the 5 end of the intron as shown in Fig. 28-18A, step a. The reaction is a simple displacement on phosphorus, a transesterification similar to that in the first step of pancreatic ribonuclease action (Eq. 12-25). The resulting free 3-OH then attacks the phosphorus atom at the other end of the intron (step b) to accomplish the splicing and to release the intron as a linear polynucleotide. The excised intron undergoes... 

An enzyme having catalytic properties and heat stability characteristics similar to those of pancreatic ribonuclease has been purihed from calf spleen and has been found in calf Uver. However, the spleen ribonuclease had a different pH optimum and is separable from pancreatic ribonuclease on a column of cation exchange resin 166). Ribonucleases, with different specificities, have also been described in a variety of animal and plant tissues 157,168), and several ribonucleases of Aspergillus oryzae have been purified and studied 159-161). 

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