TRNA bases

Stage 2 Initiation The mRNA bearing the code for the polypeptide to be made binds to the smaller of two ri-bosomal subunits and to the initiating aminoacyl-tRNA. The large ribosomal subunit then binds to form an initiation complex. The initiating aminoacyl-tRNA base-pairs with the mRNA codon AUG that signals the beginning of the polypeptide. This process, which requires GTP, is promoted by cytosolic proteins called initiation factors. 

The tRNA molecules have single chains with between 73-93 ribonucleotides. Most of the tRNA bases are adenine (A), cytosine (C), guanine (G), and uracil (U). There also are a number of unusual bases that are methylated derivatives of A, C, G, and U. 

B. The anticodon loop of aminoacyl tRNA base pairs with its corresponding codon. 

Figure 3-12. The cloverleaf structure of tRNA. Bases that commonly occur in a particular position are indicated by letters. Base-pairing in stem regions is indicated by lines between strands, xy = pseudouridine T = ribothymi-dine D = 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. 

DNA-JV-methyladenine or 5-rrredr.ylcytDsme Methylated tRNA bases - Methylnicotinamide Methylated amino acid residues... 

The methionine anticodon (UAC) on the tRNA base pairs with the methionine codon AUG in the mRNA sequence that signals the start of protein synthesis. 

The cytokinins themselves cannot be incorporated into nucleic acids, but adenine residues in tRNA can be converted to cytokinin moieties. Roughly 0.05 to 0.1% of tRNA bases are estimated to be cytokinins. 6-(3-Methyl-2-butenylamino)purine, 6-(4-hydroxy-3-methyl-2-czs-butenyl-... 

Fig. 1. Cloverleaf diagram of yeast phenylalanine tRNA (tRNA ). Bases enclosed in solid or dashed squares are constant for all tRNAs active in chain elongation. Solid lines indicate tertiary interactions between bases and involve one, two or three hydrogen bonds. The regions a and p in the D loop contain from I to 3 nucleotides in different tRNA sequences [81T3].

The rare bases of tRNA present an interesting case of enzymatic modification. After the biosynthesis of tRNA on the DNA chain, where only the four natural bases Ade, Gua, Cyt, and Lira are incorporated, some two dozen enzymes act specifically to modify some of the tRNA bases. Some are modified on the unfolded tRNA chain, whereas others, like the methylated bases, are formed only after the tRNA has assumed its tertiary structure. It is thus not surprising to find these methylated bases in the loops. Some of these bases, such as cytokinins (e.g., isopentenyl-Ade) (16) in the anticodon loop, have long aliphatic chains, which take up considerable space. 

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