Tetrahymena pre-rRNA

Although a phosphodiester bond must be formed between the upstream and downstream exons, there is no direct ATP requirement for splicing of mRNA introns or for splicing of the Tetrahymena pre-rRNA intron. Explain this observation. 

Emerick, V. L., and Woodson, S. A. (1993). Self-splicing of the Tetrahymena pre-rRNA is decreased by misfolding during transcription. Biochemistry 32, 14062—14067. 

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... 

In 1982, Tom Cech discovered that when the pre-rRNA of the protozoan Tetrahymena was incubated with Mg2+ and guanosine monophosphate, splicing of the RNA occurred without the involvement of any protein. This ability of RNA to carry out its own splicing has given rise to the term ribozyme. ... 

Self-splicing of pre-rRNA from the protozoan Tetrahymena. The first step is a transesterification reaction in which the 3 hydroxyl group of a guanosine attacks the phosphodiester bond at the 5 splice site. The second step involves another transesterification reaction in which the 3 hydroxyl group of the upstream exon attacks the phosphodiester bond at the 3 splice site and displaces the 3 hydroxyl group of the intron. 

In Tetrahymena, the pre-rRNA molecule contains an intron that is removed by self-splicing (in the presence of guanosine, GMP, GDP or GTP) without the need for involvement of any protein. This was the first ribozyme discovered but many have since been reported. 

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