- A mature tRNA is generated by processing a precursor.
- The 5 end is generated by cleavage by the endonuclease RNAase P.
- The 3 end is generated by cleavage followed by trimming of the last few bases, followed by addition of the common terminal trinucleotide sequence CCA.
tRNAs are commonly synthesized as precursor chains with additional material at one or both ends. Figure 7.6 shows that the extra sequences are removed by combinations of endonucleolytic and exonucleolytic activities. One feature that is common to all tRNAs is that the three nucleotides at the 3 terminus, always the triplet sequence CCA, are not coded in the genome, but are added as part of tRNA processing (for review see Hopper and Phizicky, 2003).
The 5 end of tRNA is generated by a cleavage action catalyzed by the enzyme ribonuclease P.
The enzymes that process the 3 end are best characterized in E. coli, where an endonuclease triggers the reaction by cleaving the precursor downstream, and several exonucleases then trim the end by degradation in the 3 -5 direction. The reaction also involves several enzymes in eukaryotes. It generates a tRNA that needs the CCA trinucleotide sequence to be added to the 3 end.
The addition of CCA is the result solely of an enzymatic process, that is, the enzymatic activity carries the specificity for the sequence of the trinucleotide, which is not determined by a template. There are several models for the process, which may be different in different organisms.
In some organisms, the process is catalyzed by a single enzyme. One model for its action proposes that a single enzyme binds to the 3 end, and sequentially adds C, C, and A, the specificity at each stage being determined by the structure of the 3 end. Other models propose that the enzyme has different active sites for CTP and ATP.
In other organisms, different enzymes are responsible for adding the C and A residues, and they function sequentially.
When a tRNA is not properly processed, it attracts the attention of a quality control system that degrades it. This ensures that the protein synthesis apparatus does not become blocked by nonfunctional tRNAs.