Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Regulation of Gene Expression
Regulation of Gene Expression at the RNA Processing Level

Prokaryotic mRNA generally does not undergo Processing. Furthermore, METABOLISM/31.html">Transcription and Translation are coupled in prokaryotes: translation begins even before mRNA synthesis is complete. Only in rare instances do prokaryotic mRNAs undergo maturation. Consequently, Gene Expression Regulation at the level of mRNA maturation is virtually absent in prokaryotes.

Unlike prokaryotes, eukaryotic mRNA undergoes extensive modifications. A cap is added to the 5' end, and the 3' end is polyadenylated. The cap facilitates proper Translation initiation. The poly(A) tail protects the 3' end of the mRNA from degradation of the coding sequences, thereby increasing its half-life. mRNA Processing is discussed in greater detail in the section «RNA and RNA Processing».

Alternative Splicing plays a pivotal role in the Introduction/30.html">Regulation of Gene Expression at the level of mRNA maturation. Through alternative splicing, a single precursor can yield multiple mRNA variants that differ in their exon composition. Proteins translated from such mRNAs share certain Structural domains while differing in specific polypeptide chain fragments (Fig. 8.20).

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Fig. 8.20. Translation of an mRNA produced via alternative splicing yields proteins with both identical and distinct polypeptide chain fragments

The pathway of alternative splicing is regulated by specialized proteins that interact with RNA precursors within introns or at intron-exon boundaries.

Alternative splicing is of profound biological significance. It determines Sex Determination in Drosophila. Instances of alternative splicing are widespread in Muscle and nervous Tissues, as well as during the synthesis of transcription factors, etc.

RNA Editing

Following transcription, the Primary Structure of RNA can undergo editing, which involves the substitution of certain bases with others. Editing is characteristic of mitochondrial mRNA. In higher plants, 3 to 15% of mitochondrial RNA is edited, compared to up to 50% in Protozoa. RNA editing also occurs in Plastids, accounting for 0.13% of codons. Typically, this process involves The conversion of C to U, though U to C substitutions also occur. The editing machinery primarily targets coding sequences, but stop codons and intronic sequences can also be modified. RNA editing is carried out by specific Enzymes.