Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 140

The folding of an RNA molecule as a result of secondary and tertiary structure represents a noncovalent change, because only hydrogen bonds or electrostatic interactions, not chemical (covalent) bonds, are formed or broken. However, once the RNA is synihesized, RNA processing and RNA modification can introduce covalent changes.

RNA processing involves forming or breaking phosphate bonds in the RNA after it is made. For example, the terminal phosphates at the 5′ end may be removed, or the RNA may be cut into smaller pieces and even relegated into new combinations, requiring the breaking and making of new phosphate bonds. In one of the most extreme cases of RNA processing, called RNA editing, nucleotides can be excised from or added to mRNA after it has been transcribed from DNA.

RNA modification, in contrast, involves altering the bases or sugars of RNA. Examples include methylation of the bases or sugars of rRNA (ribosomal RNA) and enzymatic alteration of the bases of tRNA (transfer ribonucleic acid). In eukaryotes, “caps” of methylated nucleotides are added to the 5′ ends of some types of mRNA. In bacteria, mRNAs are not capped, and only the stable rRNAs and tRNAs are extensively modified.