BIOL 3450 Chapter Notes - Chapter 10: Nuclear Membrane, Pabpn1, Exonic Splicing Enhancer
16 February 2019
BIOL 3450
Chapter 10 – Post Transcriptional Gene Control
o Primary transcripts ~ produced from eukaryotic genes; must undergo various processing
reactions to yield corresponding functional RNAs
▪ Alternatively spliced ~ mRNAs encode related proteins with differences in
sequences that are limited to specific functional domains
10.1 Processing of Eukaryotic Pre-mRNA
o Conversion of initial primary transcript synthesized by RNA pol II into functional mRNA
▪ 5’ capping
▪ 3’ cleavage and polyadenylation
▪ RNA splicing
o Ribonucleoprotein (RNP) complexes ~ are capped and spliced as they are transcribed →
referred to as nuclear mRNPs after cleavage and polyadenylation → referred to as
cytoplasmic mRNPs after exchange of proteins and exportation to cytoplasm
Suring Splicing, snRNAs Base Pair with Pre-mRNA
o Small nuclear RNAs (snRNAs) ~ base pair with pre mRNA; splicing requires their
presence
Spliceosomes, Assembled from snRNPs and pre-mRNA, Carry Out Splicing
o Spliceosome ~ large ribonucleoprotein complex; begins with base pairing of U1 snRNA
to 5’ splice site and cooperative binding of protein SF1 to branch point A and
heterodimeric protein U2AF to polypyrimidine tract and 3’ AG of intron
Key Concepts
o In nucleus of eukaryotic cells, pre mRNAs are associated with hnRNP proteins and
processed by 5’ capping, 3’ cleavage and polyadenylation, and splicing before being
transported to cytoplasm
o Shortly after transcription initiation, capping enzymes associate with carboxy terminal
domain (CTD) of RNA pol II, phosphorylated multiple times at serine 5 of heptapeptide
repeat by TFIIH during transcription initiation
▪ Enzymes then rapidly add 5’ cap to nascent transcript when it reaches length of
about 25 nucleotides
▪ Transcription does not proceed at high rate until RNA processing factors become
associated with CTD (poised to interact with nascent pre mRNA)
o 5 different snRNPs interact via base pairing with one another and with pre mRNA to
form spliceosome
▪ Large ribonucleoprotein complex catalyzes 2 transesterification reactions that
join 2 exons and remove intron as lariat structure, which is subsequently
degraded
o SR proteins that bind to exonic splicing enhancer sequences in exons are critical in
defining exons in large pre mRNAs of higher organisms
▪ Network of interactions between SR proteins, snRNPs, and splicing factors forms
cross exon recognition complex that specifies correct splice sites
o snRNAs in spliceosome are thought to have overall tertiary structure similar to that of
group II self splicing introns
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Suring splicing, snrnas base pair with pre-mrna: small nuclear rnas (snrnas) ~ base pair with pre mrna; splicing requires their presence. Pap and stops their addition once polya tail reaches about 250 residues: excised introns and rna downstream from cleavage / polyadenylation site are degraded primarily by exosomes, multiprotein complexes that contain internal 3". 5" exonuclease: exosomes also degrade improperly processed pre mrnas. Because of alternative splicing of primary transcripts, use of alternative promoters, and cleavage at different polya sites, different mrnas may be expressed from same gene in different cell types or at different developmental stages. In rna editing, nucleotide sequence of pre mrna is altered in nucleus. In vertebrates, process is relatively rare, and only single base c to u changes have been observed, but those changes can have important consequences by altering amino acid encoded by edited codon.
