RNA processing
RNA species in cells
RNA processing
Types of introns and their removal
Types of introns
Complex organisms have intron-rich genes
Examples of introns in human protein-coding genes
Removal of introns from mRNA in the nucleus of eukaryotes
Consensus elements of human GU-AG introns
Chemistry of mRNA splicing for GU-AG introns
Three way junction at the internal (A) branch site
U1 snRNP
Spliceosome-catalyzed
splicing of GU-AG introns
Different snRNPs and proteins bind and dissociate
during splicing
RNA processing
Similarities in spliceosome-catalyzed removal of
introns and self-splicing group I and group II introns
Splicing of AU-AC introns by
“minor” spliceosomes proceeds
as for GU-AG introns
Trans-splicing joins
exons from different
RNAs
Possible errors in splice-site selection
SR proteins are essential for correct splice-site selection
SR proteins = serine, arginine-rich proteins
ESE = exonic splicing enhancers
Alternative splicing results in alternative mRNAs that
are translated into different proteins
Alternative splicing results in alternative mRNAs that
are translated into different proteins
Alternative splicing of the human slo gene
(codes for a membrane protein that regulates the flow
of potassium ions across the membrane)
Optional exons participating in alternative splicing events.
About 500 mRNA variants are produced in cells of the inner ear.
Figure 12.34 Genomes 3 (© Garland Science 2007)
Alternative splicing is regulated
Assembly of major
and minor
spliceosomes
Nonsense-mediated
transcript decay
Example: alternative splicing in Drosophila Dscam
transcripts
Example: mutual exclusive alternative splicing of exon 6 in
Drosophila Dscam transcripts
Splicing activators and repressors
Splicing activators and repressors
Exons may code for
protein domains
Exon shuffling may lead to protein diversity
Exon shuffling may be responsible for protein diversity
(chromatin-modifying enzymes of yeast, worms, flies, and
humans)
Types of introns
Removal of introns from tRNAs
Figure 12.40 Genomes 3 (© Garland Science 2007)
RNA processing
Processing of ribosomal RNA by cutting and trimming
Processing of a tRNA by cutting and trimming
Figure 12.17 Genomes 3 (© Garland Science 2007)
RNA processing
Chemical modifications in tRNAs
Figure 12.18 Genomes 3 (© Garland Science 2007)
Methylation of ribosomal RNAs
Figure 12.41a Genomes 3 (© Garland Science 2007)
Figure 12.41b Genomes 3 (© Garland Science 2007)
Table 11.3 Genomes 3 (© Garland Science 2007)
RNA processing
RNA editing in apolipoprotein B mRNA
Deamination of bases leads to codon changes
RNA-guided insertion of Us into mRNAs
Processing of RNAs is required for transport into
the cytoplasm