o Perhaps a protection against RNA viruses these viruses generate dsRNA

O perhaps a protection against rna viruses these

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o Perhaps a protection against RNA viruses: these viruses generate dsRNA intermediates when they replicate o Perhaps a safeguard against excessive movement of transposons Transposons: short segment of DNA that can move around in the genome (ie topoisomerase that allows it to replicate and leave one site and go to another site on the genome); they drive mutation and evolution When they move, they can randomly insert near promoters, and their own genes can get activated Since they insert randomly, both sense and antisense RNA can get transcribed from them; if these hybridize with each other the RNA system is activated (dsRNA produced, then can be broken down) Translational control of specific mRNAs through 3’ UTR regulatory elements cytoplasmic polyadenylation of mRNAs regulated in unfertilized egg, many mRNAs are stored with short poly(A) tails and are translationally silent until the egg is fertilized thus regulation at the poly(A) tail, requires U-rich sequence element CPE (cytoplasmic polyadenylation element) in addition to the AAUAAA nuclear poly(A) signal; CPEB (CPE binding protein) binds to CPE CPEB is expressed during oogenesis when the RNA needs to be silent; CPEB also interacts with protein Maskin Maskin binds to eIF4E The mRNA is translationally dormant because Maskin not only binds to eIF4E but it binds to the same site on eIF4E that would bind to another member of the eIF4 family essential for active cap binding complex formation Thus maskin blocks assembly of eIF4 complex, thus blocking association of these messages with the ribosome After egg fertilization, these mRNAs are activated by cytoplasmic polyadenylation CPEB is phosphorylated and does not bind to Maskin anymore
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This allows cytoplasmic forms of CPSF and PAP to bind, and allowing eIF4E to associate with eIF4G, and addition of A residues to mRNA recruited to ribosome as eIF4 complex is assembled - RNAs can also be regulated in the cytoplasm at the level of their stability o Stability of mRNA influenced by the sequences in their 3’ UTR o Most mRNAs have long half-lives of many hours o Some mRNAs have short half-lives of <30 min; these mRNAs contain multiple AU-rich elements in their 3’ UTR The AU-rich element binding proteins recruit a deadenylating enzyme (to remove poly(A) tail) and the exosome to degrade these mRNAs Regulated degradation is a mechanism used to silence expression of certain genes o Eg: regulation of IRE-BP (iron response element binding protein) IRE-BP regulates translation of ferritin mRNA (protein that binds up excess iron) by interacting with stem-loop elements in its 5’UTR (IRE, iron response elements) and blocking ribosome scanning In conditions of high iron, IRE-BP are not bound, IRE-BP is not produced, ferritin is translated In conditions of low iron, IRE-BP is activated, binds to stem-loop structures, now eIF4A cannot carry out scanning function because it can’t unwind these stem loops, leading to no translation initiation
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