Chapter 18 | Posttranscriptional Regulation in Eukaryotes

Post-Transcriptional Processing Steps

The sequence of mRNA maturation: transcription → 5' capping → 3' cleavage → polyadenylation → splicing → termination

5’ Cap

Modification that is added during a temporary stall of RNA polymerase early in transcription. This protects mRNA from exonucleases, aids ribosome positioning, assists splicing, and promotes nuclear export

3’ Cleavage

A specific endonucleolytic cut that signals transcription termination and prepares the RNA for polyadenylation

Poly-A Tail

As the poly-A tail shortens over time, the mRNA becomes unstable and is eventually removed from the cytoplasm

Introns

Non-coding, intervening sequences removed from pre-mRNA during splicing

Exons

Coding sequences retained and ligated together to form mature mRNA

mRNA Folding

Eukaryotic mRNA can fold back on itself to create structural docking sites for proteins and regulatory complexes

Untranslated Regions (UTRs)

The 5' and 3' ends of mRNA that are not translated but play regulatory roles

mRNA Subcellular Localization

Specific mRNAs (e.g., for Bicoid) are transported to particular regions of the cell for localized translation

Structure of the 5’ Cap

A guanine nucleotide linked via a 5'-5' triphosphate bridge, with methylation of bases and the 2' hydroxyls

Electron Microscopy of Introns

DNA–mRNA hybridization experiments reveal looped regions corresponding to introns removed during processing

Intron-Exon Border Consensus

Conserved sequences mark the 5' and 3' splice sites; though not identical, they define splice junctions

Pyrimidine-Rich Tract

A ~15-base region near the 3' end of the intron important for splice site recognition

Branch Point A

A conserved adenine located near the 3' splice site that serves as a critical nucleophile in splicing

Splice Site Binding Proteins

Distinct proteins bind the 5' splice site, the 3' splice site, and the pyrimidine-rich region to initiate splicing

Spliceosome

A large macromolecular complex responsible for removing introns and ligating exons

U1 snRNP

Recognizes and binds the 5' splice site of the intron

U2 snRNP

Binds the pyrimidine-rich region and the branch point A to initiate spliceosome assembly

U1-U6 snRNP Complex

A coordinated set of snRNPs that together identify the 3' splice site and organize the splicing reaction

ATP-Driven Spliceosome Assembly

ATP hydrolysis drives conformational rearrangements that allow spliceosome formation and activation

Splice Site Looping

The spliceosome brings the 5' and 3' splice sites into close proximity to enable intron excision

snRNPs (Small Nuclear Ribonucleoproteins)

RNA-protein complexes composed of snRNA and associated proteins, essential for recognizing splice sites and catalyzing splicing