Molecular Genetics

Some tRNAs are synthesized in long precursors containing several tRNAs and are released by endonucleotlytic processing.

Facts

• The T even phages have a cluster of 7 tRNA genes that are transcribed into a single precursor RNA. The processing of this precursor illustrates the steps involved in tRNA maturation.
• Comparison of the transcript with the mature tRNA structure reveals that processing steps include base modification, the removal of 5' and 3' nucleotides, and (in some cases) addition of CCA to the 3' ends.
• Mutations (often temperature sensitive) in tRNA processing genes result in the accumulation of partially processed intermediates.
• Characterization of the intermediates accumulated as the result of each mutation led to piecing together the order of processing steps.
  • (1) Some of the bases are modified (*).
  • (2) Endonucleases cut the modified transcript into tRNA-containing units.
  • (3) Exonucleolytic trimming occurs at the 3' end.
  • (4) A CCA tail is added to the 3' ends (if not already present).
  • (5) Further base modification occurs.

Interpretations

• Multiple mature RNAs may be derived from a single RNA transcript.
• RNA secondary structure and enzyme specificity are important for correct processing of precursor RNAs.

Further information

• Some RNA processing enzymes are RNA-protein complexes, in which the RNA plays the catalytic role.
• Some tRNAs are encoded in the same transcriptional unit as mRNAs or as rRNAs.
• Some tRNAs require cleavage and removal of some internal sequences.

This is page 2322 of Molecular Genetics by Ulrich Melcher, © 1998, 1999, 2000