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14

The Nature of Living Things

binding of transcription factors to promoter sites is essentially as in prokaryotes;

in eukaryotes, more proteins tend to be involved, allowing very fine tuning of

expression.

Some molecules can directly interact with mRNA, altering its conformation and

preventing translation into protein. This ability can be used to construct a simple feed-

back control mechanism; that is, the mRNA binds to its translated protein equivalent.

mRNAs able to act in this way are known as riboswitches.

14.8.5

mRNA Processing

Post-transcriptional modification, or RNA processing, refers to the process whereby

the freshly synthesized RNA is prepared for translation into protein. In prokaryotes,

translation often starts while the RNA is still being synthesized; in eukaryotes, there

is an elaborate sequence of reactions preceding translation. In summary, they are

capping; 3 prime3^'-polyadenylation; splicing; and export. Moreover, the whole process is

under molecular surveillance and any erroneously processed RNA is degraded back

into monomers.

Splicing is needed due to the introns interspersed in the DNA coding for protein.

The initially transcribed RNA is a faithful replica of both introns and exons. This pre-

mRNA is then edited and spliced (by the spliceosome, which is constituted from small

nuclear riboprotein particles (snRNPs), each incorporating five small nuclear RNAs

and several proteins bound to them). The DNA and the enzymes for transcription

and post-transcriptional modification are enclosed in the lipid bilayer-based nuclear

envelope, from which the edited RNA is exported (as messenger RNA, mRNA) into

the cytoplasm for translation. These events are coördinated by sequence-specific

RNA-binding proteins. ⁵⁷

Alternative splicing of pre-mRNA is a powerful way of generating variant proteins

from the same stretch of DNA; a majority of eukaryotic genes are probably processed

in this way and, hence, the number of different proteins potentially available far

exceeds the number of genes identified from the sequence of the genome. This

method of generating variety is especially prominent in the generation of B-cell

diversity in the immune system (Sect. 14.6).

14.8.6

Translation

The mature mRNA emerges from the nucle(ol)us where it is processed by the ribo-

somes, which are large (upper M Subscript r Baseline tilde 3 times 10 Superscript 6Mr ∼3 × 10⁶ in bacteria; eukaryotic ones are larger), abun-

dant (about 15 000 in an E. coli cell) protein–RNA complexes. In eukaryotes, ribo-

somes are typically associated with the endoplasmic reticulum, an extensive internal

57 Keene (2007).