2021/2/21 1-1 polypeptide chains are specified by ORF 3 The protein coding region of each mRNa is composed of a contiguous, non-overlapping string of codons called an opening reading frame 名(ORF) a An orF should begins with a start codon and end with stop codon a mRNa containing more than one ORF is called polycistronic mRNAS 11
2021/2/21 11 1-1 polypeptide chains are specified by ORF ◼ The protein coding region of each mRNA is composed of a contiguous, non-overlapping string of codons called an opening reading frame (ORF) . ◼ An ORF should begins with a start codon and end with stop codon. ◼ mRNA containing more than one ORF is called polycistronic mRNAs. Message RNA
2021/2/21 A U GAAAG C AAUUUUCIG UAC U GAAAG GUUG GUG G CA C UUC CUGA ⑧000③①③s 5 AUGAAAGC A UUUC GUACUG AAAGG UUG G UG G CG C AC UUC CUG A 邮p⊙0②咖(②⑥①⊙ A UG A AA G CA A UUC G UA C UG A AAG GUUGG UGG CGC ACUUCICUGA 0③③①0③0②③ Fig 14-1 Three possible reading frames of the E. coli trp leader sequence 12
2021/2/21 12 Fig 14-1 Three possible reading frames of the E. coli trp leader sequence
1-2 Prokaryotic mRNAs have a ribosome binding 3 site that recruits the B translational machinery 2 1-3 Eukaryotic mRNA are modified at their 5 and 3 ends to facilitate translation 13
2021/2/21 13 1-2 Prokaryotic mRNAs have a ribosome binding site that recruits the translational machinery Message RNA 1-3 Eukaryotic mRNA are modified at their 5’ and 3’ ends to facilitate translation
a Ribosome binding site(RBs)or SD sequence in prokaryotic mRNA complementary with the sequence at the 3′ end of16 S rRNA UCCUCC CACUAG5 16S rRNA 5nNNNGGAGGNNNNNNNAUGNN 3 stop RBS start RBS start stop RBS start stop 5 PP 5-9 bases 00000000000 00o000 Fig 14-2-a structure of MRNA 14
2021/2/21 14 ◼ Ribosome binding site (RBS) or SDsequence in prokaryotic mRNA, complementary with the sequence at the 3’ end of 16S rRNA. Fig 14-2-a structure of mRNA
Kozak sequence 5NN8 NNNAUGGNN3′ start op 5 AUG AAAAA 3 5 cap Fig 14-2-b a Eukaryotic mRNa uses a methylated cap to recruit the ribosome. Once bound the ribosome scans the mrna in a 5 -3 direction to find the aug start codon a Kozak sequence increases the translation efficiency a Poly-A in the 3 end promotes the efficient recycling of ribosomes
2021/2/21 15 Once Kozak sequence ◼ Eukaryotic mRNA uses a methylated cap to recruit the ribosome. Once bound, the ribosome scans the mRNA in a 5’-3’ direction to find the AUG start codon. ◼ Kozak sequence increases the translation efficiency. ◼ Poly-A in the 3’ end promotes the efficient recycling of ribosomes Fig 14-2-b