BIOL 2960 Lecture 35: Eukaryotic Translation

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Washington University in St. Louis
Biology And Biomedical Sciences
Biology And Biomedical Sciences BIOL 2960
Kunkel Barbara

Lecture 35-36 Monday, April 24, 2017 2:45 AM EUKARYOTIC TRANSLATION, REGULATION, and POST-TRANSLATIONAL TRAFFICKING AND MODIFICATION • Eukaryotic translation ○ Mechanism ▪ Very similar to translation in prokaryotes! ▪ Major difference: translational initiation □ Prokaryotes: Shine-Delgarno sequence positions ribosomes and chooses initiator codon; formylated methionine used as the initial AA □ Eukaryotes: 1) 5' 7-methyl-guanosine cap on mRNA transcript binds initiation factors (eIF's) and recruits 40S subunit with additional eIF's + GTP and met-tRNA to 5' end of message, 2) scans to first AUG and 60S subunit binds to 40S subunit; GTP is hydrolyzed and eIFs are released, 3) Initiation; no f-met in eukaryotes, just methionine -- scanning requires ATP ○ Regulation ▪ Global □ Prokaryotes: regulated by access to the Shine-Delgarno Sequence. Repressors cover up the SD sequence; activators make it accessible. One gene at a time. □ Eukaryotes: global regulation through translational factors. Many genes at a □ Eukaryotes: global regulation through translational factors. Many genes at a time -- microRNAs □ In eukaryotes there are two types of regulation:  Global regulation through inhibition of initiation factor activity. One major target is the factor that binds the 5' cap structure  Regulation of groups of mRNAs through generation of small RNAs called microRNAs or miRNAs that bind to sequences in the 3' UTFs of target genes and inhibit their translation ▪ microRNAs □ miRNA Biogenesis and Function  miRNA transcribed by PolII as a precursor for capped, polyadenylated RNA -- non-coding RNAs are processed into miRNAs  Drosha -- a nuclease releases hairpin structure in nucleus  Dicer -- a nuclease cuts off loop; releases double stranded RNA in cytoplasm  RISC -- RNA Induced Silencing Complex -- chooses one of the strands and presents it to mRNA for complementary base-pairing and translation repression -- represses translation □ MicroRNAs are small single-stranded RNAs (~20 bp) which are processed out of longer RNA Pol2 transcribed RNAs (pre-miRNAs) by two different processing nucleases □ Ultimately they bind to a protein that "presents" them for complementary base- pairing to target sequences, typically within the 3' untranslated regions of mRNAs □ If base pairing is perfect, the mRNA will be cut by an associated nuclease activity. If base pairing is imperfect, the mRNA will be made unavailable for translation. Human miRNAs are probably exclusively of the latter. □ There are many different miRNAs. Because each miRNA type can target the 3' UTRs of mRNAs for more than one gene, miRNAs provide a way to regulate networks of genes (in addition to the networks set up by transcriptional activator proteins). □ Possible models for the repression of translation by miRNA: • Protein trafficking and modification ○ What happens to proteins aft
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