Question

Describe the features present in a mature eukaryotic messenger RNA (mRNA) and discuss how they facilitate translation initiation

Answer 1

Eukaryotic mRNA are mostly monocistronic ,having an average size of 1500 to 2000 nucleotides. It has 5 end cap, which is recognized by small ribosomal subunit. Eukaryotic mRNA molecules often require extensive processing and transport.

The intitiator tRNA s called tRNA_i^Met. In the process of initiation,initiation factors eIF₂ and eIF₃ bind to the 40S ribosomal subunit. In GTP bound form(eIF₂ GTP) it binds to tRNA_i^Met, called ternary complex. Binding of eIF₃ and ternary complex with 40S ribosomal subunit gives 43S initiation complex.This complex is independent of presence of mRNA. Initiation factor eIF4F bnds to mRNA. eIF4F s a heterotrimer consisting of eIF4E,eIF4G,eIF4A. eIF4E acts as scaffold protein,which links other components of initiation complex. eIF4A acts as helicase that unwinds any secondary structure at the 5 end. Subsequently,factors eIF4B joins eIF4F- mRNA complex. the eIF4F- mRNA-eIF4B complex join 43S initiation complex through protein protein interaction.

43S initiation complex then translocates along mRNA ,an ATP dependent process called scanning ,until it encounter the AUG initiation codon.it is assisted by factor eIF1 and eIF1A.the small subunit stops when it reaches the initiation site and form 48S complex. eIF5 or eIF5A, a GTPase activating protein,joins the growing assembly. It induce GTP hydrolysis by eIF2. Hydrolysis cause the release of factors eIF2 and eIF3. finally the factors eIF5B enable the 60S subunit to join the complex.Before eIF2 can participate in another round of initiation , its bound GDP must be replaced with a GTP. This process is catalyzed by eIF2B,a guanine nucleotide exchange factor(GEF).