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made from the death capmushroom and is a known potentinhibitor of RNA polymerase II One single mushroom could veryeasily lead to a fast death in 10days The mechanism of action is thatalpha amanitin inhibits RNApolymerase II at both the initiationand elongation states oftranscription CHAPTER 14: RNA MOLECULES AND RNA PROCESSINGRNA Molecules and RNA ProcessingIn prokaryotes, the coding region of a geneisn’t interrupted: the sequence of the gene isco-linear with the amino acid sequence ofthe protein Therefore, the prokaryotic messenger RNAsequence corresponds to the gene fromwhich it was transcribed
The Shine-Dalgarno sequence, 5’ UAAGGAGGU 3’ is involved in the initiation of translation However, in eukaryotes, genes are often interrupted: exonsare protein coding segments, introns are intervening (non-coding) segments The presence of introns in eukaryotic genes was discovered in the 1970s The removal of introns along with additional RNA processing steps are required to form the mRNA that will be translated into a polypeptide Summary of the three main processing steps in eukaryoticnuclear pre-mRNA1.Addition of 7-Methyl Guanosine CapoLinked to pre-mRNA by a unique 5’-5’phosphate linkage 3’5’Intron removal
2.Addition of PolyA Tail oPre-mRNA is cleaved and then, a long string of A residues is added by Poly A polymerase 3.Removal of Introns oIntrons in pre-mRNA are removed by a specialized process called RNAsplicing Details of the three pre-mRNA processing steps 1.The 7-Methyl Guanosine (7-MG) Cap oAddition of 7-MG cap occurs early in the elongation process 2.Addition of the 3’ PolyA Tail oEukaryotic pre-mRNA is cleaved 11-30 nt following the 5’ AAUAAA 3’ sequence in pre-mRNA oPoly A polymerase adds a string of ~200 A residues at the cleaved end 3.Removal of Introns from pre-mRNA oRemoval of introns must be precise in order to properly fuse the 3’ end of one exon to the 5’ end of the next exon oEvery intron has two conserved sequences that are required for its removal5’ and 3’ splice sites: GU and AG sequences, respectively Intron Branch Point: conserved A residue oAccomplished by an RNA/protein complex known as the spliceosome RNA/protein structure Five small nuclear RNAs (snRNAs): designated U1, U2, U4, U5, and U6The snRNAs associate with about 40 small proteins to form small nuclear ribonucleoproteins (snRNPs) snRNPs U1, U2, U4/6, and U5 assemble to form a complete spliceosome Lariat formation involves unique linkage between the 5’ snRNP assembly
phosphate of the G and the 2’ OH of the A So, the introns of nuclear pre-mRNA transcripts are carried outby spliceosomes The introns of some rRNAprecursors are removedautocatalytically by reaction of theRNA molecule itself The introns of tRNA precursorsare excised by preciseendonucleolytic cleavage andligation reactions The presence of introns allows foralternative modes of splicing and