【正文】 539。 splice site and U2 snRNP bound to the branchpoint sequence. Can a mutation in an intron affect the gene expression ? Alternative Splicing (The same gene can be spliced differently) ALTERNATIVE SPLICING IN THE MOUSE IMMUNOGLOBULIN HEAVY CHAIN mRNA Weaver Fig. Gene structure RNAs are not spliced differently in the same cell at the same time! SELFSPLICING OF TETRAHYMENA rRNA PRECURSOR STRUCTURE OF TETRAHYMENA rRNA PRECURSOR WEAVER: FIG. Selfsplicing (Group I) No lariat RNA Comparison of the spliceosome with the group II intron WEAVER: FIG. Selfsplicing (Group I) Selfsplicing (Group II) TRANSSPLICING ENTAILS THE LIGATION OF EXONS FROM TWO SEPARATE TRANSCRIPTS。 splice site recognition ? U2: branch site recognition ? U4: a chaperone for U6 ? U5: 539。539。 recognition by facilitating the interactions between U1 snRNP bound to the upstream 539。 and 339。PPP linkage of a guanosine. ? occurs cotranscriptionally ? may be required for stability of the mRNA export from the nucleus mRNA splicing translation. Capping of the 5’ end of nascent RNA transcripts with 7methylguanylate (m7G). m mRNA Polyadenylation* * Note: Most eukaryotic mRNAs contain 20250 A at 339。crossintron39。 splice site recognition ? U6: likely to be catalytic moiety, U4/U6 to U6/U2 brings U6 to branch site Basepairing between premRNA, U1, and U2 U1 snRNA basepairs with 539。 ends, a major exception are histone mRNAs. A few procaryotic mRNAs contain poly(A) tails. 5’ …. AATAAA..(1120bp)…CA….TTGTGTGTTG..3’ 3’ …. TTATTT………...…GT….AACACACAAC..5’ 5’ …. AAUAAA…( 1120b)…CA….UUGUGUGUUG..3’ Polyadenylation signal Cleavage site polyA adddition site GU rich 典型 的聚腺苷化位點 DNA RNA A MODEL OF THE PRECLEAVAGE COMPLEX CPSF: CLEAVAGE AND POLYADENYLATION SPECIFICITY FACTOR CstF: CLEAVAGE STIMULATION FACTOR CF I/II: CLEAVAGE FACTORS PAP: POLY(A) POLYMERASE WEAVER: FIG. Function of Polyadenylation ? Transcription termination ? Stability of mRNAs in eukaryotes ? translation efficiency ? Element associated with 339。 recognition plex. SR proteins also function in 39。 splice site of premRNA。 terminal intron removal EFFECT OF POLYADENYLATION ON TRANSLATABILITY AND STABILITY (a) TRA