2. RNA: Posttranscriptional Modifications By Amr S. Moustafa, M.D.; Ph.D.

3. Enhancers and Silencers Cis-acting DNA sequences Enhancers: stimulate; Silencers: inhibit Upstream or downstream Close or thousands bp from promoter DNA response elements: activators-binding sites On either strand of DNA Bending of enhancer DNA allows activators (specific TF) to interact with: General TF and RNA polymerase II

4. Enhancers and Silencers Cis-acting DNA sequences Enhancers: stimulate; Silencers: inhibit Upstream or downstream Close or thousands bp from promoter DNA response elements: activators-binding sites On either strand of DNA Bending of enhancer DNA allows activators (specific TF) to interact with: General TF and RNA polymerase II

5. Transcription Factors General: Binds to promoter Activators: Binds to Enhancer

6. The Lac Operon Operon: Coordinately expressed genes Structural and their regulatory genes Lac operon: (For E. coli catabolism of lactose) Structural genes: code for 3 enzymes: Lac Z gene: β-galactosidase Lac Y: permease Lac A: thiogalactoside transacetylase Regulatory Sequences: Promoter (P) region Operator (O) site CAP-binding site LacI gene: repressor protein

7. The Lac Operon Operon: Coordinately expressed genes Structural and their regulatory genes Lac operon: (For E. coli catabolism of lactose) Structural genes: code for 3 enzymes: Lac Z gene: β-galactosidase Lac Y: permease Lac A: thiogalactoside transacetylase Regulatory Sequences: Promoter (P) region Operator (O) site CAP-binding site LacI gene: repressor protein

8. The Lac Operon: Off

9. The Lac Operon: On

10. The Lac Operon: Off

11. Posttranscriptional Modifications The rRNA and tRNA: Both pro- and eu-karyotes The mRNA: Only eukaryotes

12. Posttranscriptional Modifications: The rRNA Ribonucleases Site: Nucleolus Protein association: Before and during modifications e 5 S rRNA: Separate modification

13. Posttranscriptional Modifications: The tRNA - 1 1. Ribonucleolytic cleavage: 16-Nucleotides at 5’-end 2 Uracil at 3’-end 14 Nucleotides (intron) at anticodon loop 2. Addition of CCA at 3’-end Nucleotidyltransferase 3. Base modifications

14. Posttranscriptional Modifications: The tRNA - 2

15. Posttranscriptional Modifications: The euokaryotic mRNA - 1 1. 5’- Capping: 7-Methyl-guanosine 2. 3’- Poly-A tail 3. Removal of introns and splicing of exons

16. Posttranscriptional Modifications: The euokaryotic mRNA - 2 Cap: 7-Methylguanosine triphosphate Bond: Unusual 5’-5’ triphosphate Site: Nucleus & cytoplasm Enzymes: Nuclear guanylyltransferase Cytoplasmic methyl transferase Methyl donor: SAM Functions: Initiation of translation Stabilizes mRNA

17. Posttranscriptional Modifications: The euokaryotic mRNA - 3 3’- Poly-A tail: 40 – 200 Exception: mRNA for histones and interferons Enzyme: Nuclear polyadenylate polymerase Site: Nucleus & cytoplasm Signal: AAUAAA Functions: Stabilizes mRNA Facilitates its exit from nucleus Poly-A tail: Shortened after mRNA enters cytosol

18. Posttranscriptional Modifications: The euokaryotic mRNA - 4

19. Posttranscriptional Modifications: The euokaryotic mRNA - 5 Removal of introns and splicing of exons: Exons:Coding sequences for proteins Spliceosome: Binding of snRNPs & primary transcript Correct alignment of neighboring exons allows splicing of exons The snRNPs: snRNAs + proteins Introns:Sequences not coding for proteins No, few or many introns/mRNA

20. Posttranscriptional Modifications: The euokaryotic mRNA - 5 Systemic lupus Erythematosus: Auto Abs against snRNPs β-Thalassemia: Splice site mutations

21. Posttranscriptional Modifications: The euokaryotic mRNA - 6 Alternative splicing: A diverse set of proteins from a small set of genes Selective tissue expression e.g., tissue-specific tropomycin