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Some of the RNAs in cells are modified after they are made. For example, specific nucleotides in the rRNAs are methylated. The tRNAs are probably the most highly processed and modified RNAs in cells (see Björk and Hagervall, Suggested Reading). Figure 2.17 shows a “mature” tRNA that was originally cut out of a much longer molecule that may also have included the rRNAs as well as other tRNAs. Some of the bases within the tRNA are modified by specific enzymes that produce altered bases, such as pseudouracil and dihydrouracil. A CCA sequence is found on the 3′ end of all tRNAs. In some cases this is encoded in the gene, and therefore is part of the precursor RNA, while in others it is added posttranscriptionally by an enzyme called CCA nucleotidyltransferase.

Figure 2.17 Structure of mature tRNAs. (A) Standard clover leaf representation of tRNA, showing the base pairing that holds the molecule together and some of the standard modifications. D is the modified base dihydrouridine. tRNAs also contain thymine (T) and pseudouracil (Ψ) among other modifications. The amino acid is attached to the terminal A of the CCA at the 3′ end. (B) Folding of tRNA into its tertiary structure. The discriminator base (black dot), immediately upstream of the CCA, is important for tRNA recognition by the correct aminoacyl-tRNA synthetase (aaRS).