10-Biochemistry-Lecture10.pdf

Lecture 10 Post-transcriptional processing of RNA Additional material for this lecture may be found in: § Lehninger’s Biochemistry (8th ed), chapter 26: p 972-988 RNA METABOLISM - DNA-Dependent Synthesis of RNA (see Lecture 9) - Post-transcriptional processing of RNA (this lecture, 10) - RNA-Depende...

Lecture 10 Post-transcriptional processing of RNA Additional material for this lecture may be found in: § Lehninger’s Biochemistry (8th ed), chapter 26: p 972-988 RNA METABOLISM - DNA-Dependent Synthesis of RNA (see Lecture 9) - Post-transcriptional processing of RNA (this lecture, 10) - RNA-Dependent Synthesis of RNA and DNA (next lecture, 11) Learning objectives: – RNA processing: Capping and splicing (10) – RNA-dependent synthesis of RNA and DNA (11) Retroviruses and reverse transcriptase Telomerase RNA PROCESSING PROCESSING OF mRNA − OVERVIEW Dozens of proteins coordinate with each other and with proteins involved in RNA transport to the cytosol and delivered to ribosomes Processing includes: – Adding a 5’-cap – Adding a 3’-poly(A) tail – Splicing out introns and rejoining any exons for a continuous sequence – Degradation FORMATION, PROCESSING AND MATURATION OF THE PRIMARY TRANSCRIPT IN EUKARYOTES The 5′ cap (red) is added before synthesis of the primary transcript is complete. The noncoding sequence (intron) following the last exon (in orange) contains a cleavage signal and is used to attach the polyA tail. Splicing can occur either before or after the cleavage and polyadenylation steps. All the processes shown here take place in the nucleus. EUKARYOTIC mRNAs ARE CAPPED AT THE 5’ END: THE 5’ CAP (7-METHYLGUANOSINE) OF mRNA 7-Methylguanosine (m7G in green) is joined to the 5′ end of almost all eukaryotic mRNAs in an unusual 5′,5′triphosphate linkage. The 5′ end may include additional methylation. Methyl groups (pink) are often found at the 2′ position of the first and second nucleotides (derived from Sadenosyl-methionine, SAM). The 5’ cap: - Protects RNA from degradation by nucleases. - Stabilizes the mRNA by binding an initiation factor (see protein synthesis). - signal the start of translation by forming a binding site for the ribosome for translation. Generation of the 5′ cap Synthesis of the cap is carried involves four to five separate steps (adoHcy is out by enzymes tethered to the CTD of Pol II. The cap remains S-adenosylhomocysteine). tethered to the CTD through an association with the capbinding complex (CBC). INTRONS ARE FOUND IN MOST GENES Most genes in vertebrates, some in yeast, a few bacteria have introns Exons usually