Chapter 18: Posttranscriptional Regulation in Eukaryotes

A critical mechanism occurs in the nucleus via alternative splicing, where pre-mRNAs are processed in varying ways to produce distinct mature mRNA spliceforms. This process drastically increases the functional complexity of the organism’s proteome, enabling a single gene to encode multiple protein isoforms that may have specialized activities. Alternative splicing involves numerous patterns, including the inclusion or exclusion of cassette exons, intron retention, or the utilization of alternative polyadenylation sites, and is tightly regulated by the interaction between cis-acting sequences (like splicing enhancers/silencers) and trans-acting RNA-binding proteins (RBPs). Defects in this process are linked to spliceopathies such as Myotonic Dystrophy and Spinal Muscular Atrophy (SMA). Following nuclear processing, the expression level of mRNAs in the cytoplasm—their steady-state level—is controlled by their stability and rate of degradation. mRNA decay pathways, predominantly deadenylation-dependent decay, begin with the shortening of the protective poly-A tail by deadenylases, often followed by removal of the 5′ cap by decapping enzymes, making the transcript vulnerable to degradation by exonucleases like XRN1. This stability is regulated by RBPs binding to sequences such as AU-rich elements (AREs). Cells also utilize surveillance pathways like nonsense-mediated decay (NMD) to efficiently eliminate mRNAs containing premature stop codons. A powerful and highly specific layer of control is managed by noncoding RNAs (ncRNAs), which mediate RNA interference (RNAi). Small ncRNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs), are processed by the enzyme Dicer and associate with the RNA-induced silencing complex (RISC). Depending on the degree of complementarity between the ncRNA guide and the target mRNA, RISC either cleaves and degrades the mRNA (perfect match) or inhibits its translation (partial match). Furthermore, long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) can act as competing endogenous RNAs (ceRNAs) or "sponges," sequestering miRNAs away from their targets to promote gene expression. Gene expression is also regulated at the translational level by controlling initiation and by mRNA localization. Some mRNAs are stored in an inactive state, often requiring cytoplasmic polyadenylation (lengthening of the poly-A tail) triggered by kinases before translation can begin. Localization ensures proteins are synthesized in specific cellular regions—for instance, actin mRNA transport guided by RBPs binding to a "zip code" sequence. Finally, after translation, proteins undergo posttranslational modifications (PTMs). The most common PTM is reversible phosphorylation, catalyzed by kinases and phosphatases, which alters a protein’s conformation and activity. Proteins are also targeted for destruction via ubiquitin-mediated protein degradation, where poly-ubiquitin chains are covalently attached by ubiquitin ligases, tagging the protein for unfolding and cleavage by the proteasome.