RNA Processing 2 (4)

Role of snoRNA or small nucleolar RNAs

To direct ribosomal RNA modifications

What other RNA is also extensively modified?

tRNAs

How are tRNAs modified?

1. Removal of the 5’ sequence

2. Removal of a short segment

3. Addition of CCA towards 3’

4. Modification of other bases inside

Common RBPs

RNA recognition motif (RRM) and Polypyrimidine tract binding protein (PTB)

How is splicing made more efficient?

By U2AF and SR proteins

What forms the cross-exon recognition complex?

SR protein:protein/snRNA interactions

U2-U2AF-SR proteins-U1

U2AF

Between the branch point and the splice acceptor

SR proteins bind to…

Exonic splicing enhancers = helps U1 and U2 find their true splice site and branch point

Alternative splicing

Formation of different proteins from the same gene by splicing out exons

Sxl RBP in Drosophila

Determines if male or female traits will be expressed

Sxl in early embryogenesis

Expressed only in females

Particularity of exon 3

Contains an inframe stop site for the production of Sxl protein

How does Sxl protein help females make more of it?

By sitting down on the presumptive link between exons 2 and 3 (high affinity) = U2AF cannot identify it as an exon = exon 3 is alternatively spliced

What other sequence does Sxl protein activate in females?

Tra protein expression sequence (1-3 vs 1-2-3)

Tra sitting on SR proteins on ESE from exon 4

Leads to the expression of female Dsx proteins = female traits (123-4 vs 123-5-6)

Alternative splicing in cochleal channels

576 different isoforms = allows sensitivity gradients

RNA editing in higher eukaryotes

Deamination (A→I and C→U)

What do the deaminations result in?

Unvoluntary shortening of proteins (CAA→UAA)

Which pre-mRNAs are not polyadelynated?

Histone pre-mRNAs

Are histone mRNAs therefore rapidly degraded?

No unlike other mRNAs

Polyadenylation signal

AAUAAA…G/U recognized by cleavage and polyadenylation factors

2nd step

Cleavage

3rd step

PAP catalyses the formation of poly A₁₂ tail (slow phase)

4th step

PABPN1 catalyses the addition of 200 A residues (rapid phase)