12.1: Processing of Eukaryotic Pre-mRNA

• Three major events:

  • 5’ capping

  • Once nascent molecules reach up to 25 to 30 molecules in length, 7-methylguanosine is added to the 5’ cap along with any other components that can be found on eukaryotic mRNA’s.

  • 3’ cleavage/polyadenylation

• 360-kDa (a cleavage and polyadenylation factor; made of four polypeptides) forms an unstable connection with the AAUAAA poly(A) signal.

• Then, three additional proteins bind to a CPSF-RNA complex, which later interacts with the Glurich sequence (another cleavage factor), and a poly(A) and polymerase (PAP) bind to the complex before cleavage starts.

• RNA splicing

• Introns are removed and it results in an A residue that is called a branch point.

12.2: Regulation of Pre-mRNA Processing

• Different mRNAs can show the same gene during different development stages or in different types of cells because primary transcripts and the cleavage at different poly(A) sites get alternatively spliced.

• RNA- binding proteins can regulate alternative splicing by binding the proteins near splice sites that are regulated.

• Splicing factors become enhanced due to the splicing activators interacting with them, so their interaction with the regulated splice site activates.

• The pre-mRNA in the nucleus has its nucleotide sequence altered during RNA editing, though this doesn’t happen often invertebrate animals. When it does occur, it causes a change in the amino acids and results in proteins with different functions.

12.3: Macromolecular Transport Across the Nuclear Envelope

• Nuclear envelopes have nuclear pre complexes with many functions:

  • They are large structures with 50 to 100 nucleoporins (a type of protein)

  • FG- nucleoporins (contain a hydrophobic sequence) help with the transport of macromolecules in nuclear pores.

  • These macromolecules can be transported easily if they are under 60 kDa, otherwise, they require the assistance of proteins.

• Proteins have a special amino acid sequence if they are imported or exported from the nucleus. These amino acid sequences form nuclear-localization signals (NLS), or nuclear-exporting signals (NES).

• NES and NLS come in different forms. They work with a specific receptor protein called karyopherins.

• Only fully functional mRNAs can be exported from the nucleus.

12.4: Cytoplasmic Mechanisms of Post-transcriptional Control

• Micro RNAs (miRNAs) can repress translations and this helps form hybrids in specific mRNAs.

• miRNAs and siRNAs are made from longer precursor molecules and help with either repressing or cleaving target mRNAs.

• Because of the degradation of mRNAs, their poly(A) tail shortens, and either a 3’ to 5’ digestion occurs or removal of 5’ cap and digestion of 5’ to 3’ occurs.

• Degradation of mRNAs in the cytoplasm happens due to the bindings of proteins in the 3’ or 5’ UTRs.

12.5: Processing of rRNA and tRNA

• Precursors of pre-mRNA that are synthesized by polymerase I undergo cleavage, which results in 28S, 18S, and 5.8S rRNAs.

• Pre-rRNA is synthesized in the nucleolus. 5S rRNA is synthesized while RNA polymerase III is not processed.

• Pre-tRNA can have some of its intron removed near the 5’ sequence.

• RNA molecules, in the nucleus and after getting exported from the cytoplasm, are associated with proteins.