TRNA posttranscriptional modification

A. Primary tRNA transcript. B. Functional tRNA after posttranscriptional modification. Modified bases include D (dihydrouridine), (pseudouridine), and m. which means that the base has been methylated. 

An intron (see below) must be removed from the anticodon loop, and sequences at both the 5 - and the 3 -ends of the molecule must be trimmed. Other posttranscriptional modifications include addition of a -CCA sequence by nucleotidyltransferase)to the 3 -terminal end of tRNAs, and modification oTbases at specific positions to produce "unusual bases (see p. 290). 

All tRNAs contain several ribonucleotides that differ from the usual four (12 in the case of tRNAphe). The structures for some of these are shown in figure 28.4. Only four ribonucleotides are incorporated into RNA in the transcription process. All of the rare bases found in the mature tRNA result from posttranscriptional modification. 

Be able to discuss the posttranscriptional modification and processing of RNAs (rRNA, mRNA, and tRNA). 

Kawai G, Yamamoto Y, Kanrimura T, Masegi T, Sekine M, Hata T, et al. Conformational rigidity of specific pyrimidine residues in tRNA arises from posttranscriptional modifications that enhance steric interaction between the base and the 2 -hydroxyl group. Biochemistry 1992 31 1040-1046. 

Figure 5-22. Synthesis of tRNA. D, T. y, and are unusual nucleotides produced by posttranscriptional modifications.

Posttranscription modification of tRNA The synthesis of tRNA involves modification of some uridine nucleotides to unusual nucleotides, such as pseudouridine, ribothymidine, and dihydrouridine. 

Posttranscriptional processing of primary RNA transcripts includes RNA splicing, 50 capping, 30 polyadenylation, and tRNA base modifications. Many of these alterations increase RNA stability and enhance inRNA translation. 

FIGURE 11.32 Posttranscriptional modification of a tRNA precursor. Dashes represent hydrogen-bonded base pairs. The symbols Gqh, Cqh. Aqh> and Uqh refer to a free 3 end without a phosphate group is a methylated guanine. 

Posttranscriptional modification All tRNA have a large fraction (up to 25%) of modified bases. Most common modification is methylation catalyzed by methyltrans-ferases using SAM as a methyl donor to yield various methylated bases such as m C, m A, m G and dim G. 

The sequences of all three pieces of RNA in the E. coli ribosomes are known as are those from many other species. These include eukaryotic mitochondrial, plas-tid, and cytosolic rRNA. From the sequences alone, it was clear that these long molecules could fold into a complex series of hairpin loops resembling those in tRNA. For example, the 16S rRNA of E. coli can fold as in Fig. 29-2A and eukaryotic 18S RNA in a similar way (Fig. 29-4).38/39/67 69 The actual secondary structures of 16S and 18S RNAs, within the folded molecules revealed by X-ray crystallography, are very similar to that shown in Fig. 29-2A. Ribosomal RNAs undergo many posttranscriptional alterations. Methylation of 2 -hydroxyls and of the nucleic acid bases as well as conversion to pseudouridines (pp. 1638-1641) predominate over 200 modifications, principally in functionally important locations that have been found in human rRNA.69a... 

Posttranscriptional processing of tRNA requires several distinct steps, as summarized in Figure 25.8. First, the 50 and 30 ends must be cleaved to release the tRNA sequence from the larger precursor transcript and introns must be removed if they are present. Second, the required CCA charging sequence at the 30 end of tRNA must sometimes be added by a nucleotidyl transferase. Third, all tRNAs contain a large number of modified bases which result from reductions, methylations, and deaminations. These modifications can affect codon recognition by the tRNAs during protein synthesis (Chapter 26). 

Second, many of the bases in tRNAs are modified posttranscriptionally some of these modifications are uniform among all tRNAs, whereas others are specific for each tRNA. Third, the structural data reveal that there are four intrastrand RNA-RNA helices which are formed by standard (G-C, A-U) and nonstandard (G-U) base pairing. 

Mature tRNAs are derived from precursors (pre-tRNAs) by posttranscriptional processing, which consists of extensive base modifications, base transpositions, and the removal of both 5 leader and 3 trailer sequences. 

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