TRNA precursors pathways

Figure 28-21 Two related splicing pathways for tRNA precursors. (A) Pathway followed in yeast. (B) Related pathway used by HeLa cells.598 602...

The synthesis in many extant organisms of these two amide residues from their respective precursors glutamate and aspartate esterified to tRNA (the indirect aminoacylation pathways described in Sections 5.14.3 and 5.14.4) and that of other amino acid residues, such as selenocysteine (which is also synthesized from a precursor esterified on a tRNA °) support the model of prebiotic metabolism taking place at the surface of solid particles, " analogous to ancestral RNAs. 

Dimethylallylpyrophosphate (DMAPP) originating from cytosolic mevalonate (JVIVA) pathway probably serves as a precursor of tRNA isoprenylation [80]. Release of cytokinins via tRNA degradation could at least serve as a possible source of cA-zeatin, which is much more abundant in RNA than the tran5-isomer [89] and further de novo biosynthesis of this cytokinin species [90] as well as its isomerization from trans-xQ-dim have not been proved yet. 

Plants, in common with microorganisms and animals, require methionine chiefly for three roles, (a) As a component of protein, a role which accounts for most of the methionine in the cell, (b) As a component of methionyl tRNA (in eukaryotes) and formylmethionyl tRNA (in chloroplasts, mitochondria, and prokaryotes), factors required for initiation of protein synthesis. (c) As a component of AdoMet, the chief biological methyl donor, the obligatory precursor of spermidine and spermine, and an effector of certain enzymes. In addition to these chief roles, a major pathway for the metabolism of methionine in certain plant tissues is its conversion to ethylene (see Yang and Adams, this series, Vol. 4, Chapter 6). Only plants and microorganisms can synthesize the homocysteine moiety of methionine novo, and the importance of this synthesis in the sulfur cycle has been noted in the introduction. 

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