Copyright@The McGraw-Hill Companies,Inc.Permission required for reproduction or display. Chapter 15 Messenger RNA Processing ll: Capping and Polyadenylation K.G.Murti/Visuals Unlimited
Chapter 15 Messenger RNA Processing II: Capping and Polyadenylation
15.1 Capping By 1974,mRNA from a variety of eukaryotic species and viruses were found to be methylated(甲基化的) A significant amount of this methylation was clustered at the 5'-end of mRNA This methylation cluster formed a structure we call a cap 15-2
15-2 15.1 Capping ✓ By 1974, mRNA from a variety of eukaryotic species and viruses were found to be methylated (甲基化的) ✓ A significant amount of this methylation was clustered at the 5’-end of mRNA ✓ This methylation cluster formed a structure we call a cap
Cap Structure Early study used viral mRNA as they are easier to purify and investigate The B-phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA -Cap is at the 5'-terminus of RNA -The cap is made of 7-methylguanosine(7- 甲基化鸟甘酸，m7G) -Linkage is a triphosphate(三磷酸盐键) Charge on the cap area is near-5 15-3
15-3 Cap Structure ✓ Early study used viral mRNA as they are easier to purify and investigate ✓ The -phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA – Cap is at the 5’-terminus of RNA – The cap is made of 7-methylguanosine(7- 甲基化鸟甘酸，m7G) – Linkage is a triphosphate (三磷酸盐键) – Charge on the cap area is near -5
CHa Charges +1 Reovirus Cap Structure √The m7G contributes a positive charge OH OH Triphosphate linkage contributes 3 negative charges √Phosphodiester bond contributes 1 negative charge 0 √Terminal phosphate contributes 2 negative O OH(Methlyated in cap 2) charges -2 15-4 Net:-5
15-4 Reovirus Cap Structure ✓ The m7G contributes a positive charge ✓ Triphosphate linkage contributes 3 negative charges ✓ Phosphodiester bond contributes 1 negative charge ✓ Terminal phosphate contributes 2 negative charges
Cap Synthesis First step pppXpY. RNA triphosphatase -RNA triphosphatase(核苷磷酸 (a ppXpY-. 水解酶)removes terminal Gppp一 phosphate from pre-mRNA (b) Guanylyltransferase -Then,guanylyl transferase( GpppXpY. AdoMet- 苷酸转移酶)adds capping GMP (c) Methyltransferase from GTP m7GpppXpY. √Next,2 methyl transferases(甲 AdoMet- 基转移酶)methylate N7of (d) Methyltransferase capping guanosine and 2'-0- m7Gpppx"pY. B methyl group of penultimate (e) nucleotide CH2 This occurs early in transcription,before chain is OHOH 30 nt long S-腺苷甲硫氨酸（AdoMet) 15-5
15-5 Cap Synthesis ✓ This occurs early in transcription, before chain is 30 nt long ✓First step –RNA triphosphatase (核苷磷酸 水解酶) removes terminal phosphate from pre-mRNA –Then, guanylyl transferase (鸟 苷酸转移酶) adds capping GMP from GTP ✓ Next, 2 methyl transferases (甲 基转移酶) methylate N7 of capping guanosine and 2’-Omethyl group of penultimate nucleotide S-腺苷甲硫氨酸（AdoMet）