Eukaryotes tRNA biosynthesis

Metabolism of Macromolecules mRNA biosynthesis (bacteria) mRNA biosynthesis (eukaryotes) tRNA and rRNA biosynthesis (bacteria)... 

Protein biosynthesis (eukaryotes) Aminoacyl-tRNA biosynthesis... 

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. °... 

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... 

With regard to selenocysteine biosynthesis in archaea and eukarya, a tRNA has been known for some time, which accepts L-serine and possesses an anticodon complementary to UGA. It shares a number of structural featmes with tRNA from E. coli, such as extended aminoacyl acceptor and D-arms. In eukarya, the serine residue attached to this tRNA can be phosphorylated by a specific kinase and it was first assumed that this tRNA inserts phosphoserine into proteins. However, closer examination revealed that this tRNA carries selenocysteine in vivo and certainly is the pendant to tRNA from E. coli. It is still elusive whether (9-phosphoseryl-tRNA is the biosynthetic intermediate for selenocysteyl-tRNA formation in eukaryotes also, there is no evidence yet of an eukaryal and archaeal enzyme, equivalent in its function to selenocysteine synthase from bacteria. 

Xu XM, Carlson BA, Mix H, Zhang Y, Saira K, Glass RS, Berry MJ, Gladyshev VN, Hatfield DL. Biosynthesis of selenocysteine on its tRNA in eukaryotes. PLoS Biol. 2006 5 e4. 

Increasing evidence suggests that Met-tRNAf is also the initiator tRNA in eukaryotic systems (C35, G16). The failure of previous experiments to demonstrate the role of this Met-tRNAf for the in vitro protein synthesis is probably due to the lack of protein initiation factors. Mi, Ma, and Ms, which are present in a ribosomal salt-wash protein fraction (P24, S35, S36). The most recent experiments by Anderson and co-workers (C34, C35) show that the Met-tRNAf binds the initiation factors Ml and Ms to form an initiation complex with messenger RNA. The binding of this complex requires CTP and Mg + ions. A methionyl-valine dipeptide production is the next step in the biosynthesis of the chain the synthesis of this bond requires Mg + ions, an additional initiation... 

Coenzymes such as NAD, NADP and coenzyme A are concentrated in the aqueous phase of the matrix. The matrix also contains the protein synthesizing system of the M. Vertebrate M. contain Ribosomes (see) of size 50-55S, whereas the M. of other eukaryotic organisms contain 70S ribosomes. The DNA of M. is circular, histone-free, and attains a length (in mammals) of 5 pm. It contains cistrons for ribosomal RNA and tRNA, and the genes for ATPases, for 2 of the 3 subunits of cytochrome 6, and for 4 of the 7 subunits of cytochrome oxidase. The biosynthesis of all the other mitochondrial constituents appears to be under the genetic control of the cell nucleus. The genetic code and tRNA complement of M. are different from those of the cytoplasm (see Genetic code). 

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. 

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