RNA的生物合成和加工Transcription and Posttranscriptional P.ppt
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The RNA synthesized is also self-complementary and forms a stem-loop structure followed by aseries of U. U U 5’ GCCGCCAG C CGGCGGUC U U GG U U U 3’ RNA pol interacts with the structure and stops at the template The UA pairs are unstable. The newly synthesized RNA re- leases from the template. Transcription terminates. ρ dependent termination The terminator of the ρ dependent termination is not typical. It contains CA-rich region which ρ factor can recognize. ρ factor is a homo-hexamer protein factor , can bind the newly synthesized RNA and moves to the RNA-DNA hybrid region. ρ factor has helicase activity and unwinds the RNA-DNA helix depending on the energy released from ATP hydrolysis. Section 2 Posttranscriptional Processing Mature eukaryotic mRNA has experienced 5’- and 3’ end processing and splicing. hnRNA ( or mRNA ) is capped at 5’ end Most hnRNA have a cap of 7-methylguanosine triphosphate ( m7Gppp-) at the 5’ end. It is added to the 5’ end of the growing transcript of 25-30 nucleotides via 5’5’ triphosphate linkage. The cap protects hnRNA ( and mRNA ) from Rnase attack and participates in the binding of mRNA and ribosome. hnRNA ( or mRNA ) has a poly A tail at the 3’end Almost all of eukaryotic hnRNAs ( or mRNAs ) have a poly A tail of 80-250 nucleotides at the 3’end. It is enzymatically added to the primary transcript intwo stages. Cleavage The transcript is cleaved 10 to 30 nucleotides past a highly conserved AAUAAA sq ( the polyadenylation signal sq ) and within 50 nucleotides before a GU-rich sq. 10-30nts 50nts -----------AAUAAA----------------I-------------------GU-rich—
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