Prokaryotes tRNA biosynthesis

The structures of mRNAs, ribosomal subunits (Section 7.4) and initiator tRNAs. Nevertheless, in overview, the processes are not dissimilar. Since investigations into prokaryotic protein biosynthesis tend to precede those into eukaryotic systems, this text is restricted to the system in E. coli. The components required for each stage of protein biosynthesis in this organism are listed in Table 17.3. 

A very different ribonuclease participates in the biosynthesis of all of the transfer RNAs of E. coli. Ribonuclease P cuts a 5 leader sequence from precursor RNAs to form the final 5 termini of the tRNAs. Sidney Altman and coworkers in 1980 showed that the enzyme consists of a 13.7-kDa protein together with a specific 377-nucleotide RNA component (designated Ml RNA) that is about five times more massive than the protein.779 Amazingly, the Ml RNA alone is able to catalyze the ribonuclease reaction with the proper substrate specificity.780 7823 The protein apparently accelerates the reaction only about twofold for some substrates but much more for certain natural substrates. The catalytic center is in the RNA, which functions well only in a high salt concentration. A major role of the small protein subunit may be to provide counterions to screen the negative charges on the RNA and permit rapid binding of substrate and release of products.783 Eukaryotes, as well as other prokaryotes, have enzymes similar to the E. coli RNase R However, the eukaryotic enzymes require the protein part as well as the RNA for activity.784... 

Gene expression can also be regulated at the level of translation. In prokaryotes, many operons important in amino acid biosynthesis are regulated by attenuation, a process that depends on the formation of alternative structures in mRNA, one of which favors transcriptional termination. Attenuation is mediated by the translation of a leader region of mRNA. A ribosome stalled by the absence of an aminoacyl-tRNA needed to translate the leader mRNA alters the structure of mRNA so that RNA polymerase transcribes the operon beyond the attenuator site. 

Dimethylallylation of adenosine (i A) refers to the modification occurring at position A37 (3 adjacent to the anticodon) in both prokaryotic and eukaryotic tRNAs. In some of the early literature, this reaction was also known as isopentenylation, while i A was termed A -isopentenyladenosine. Several decades ago, i A was first isolated from yeast. In many organisms, it can be further thiomethylated at to form ms i A, whose biosynthetic mechanism had been discussed earlier in this chapter. i A biosynthesis is catalyzed by dimethylallyltransferase, which is encoded by the miaA gene in E. and the modS gene in yeast. °... 

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