KEY CONCEPTS:
- 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.