<P> The poly (A) - binding protein (PABP) also associates with the eIF4F complex via eIF4G, and binds the poly - A tail of most eukaryotic mRNA molecules . This protein has been implicated in playing a role in circularization of the mRNA during translation . This 43S preinitiation complex (43S PIC) accompanied by the protein factors moves along the mRNA chain toward its 3' - end, in a process known as' scanning', to reach the start codon (typically AUG). In eukaryotes and archaea, the amino acid encoded by the start codon is methionine . The Met - charged initiator tRNA (Met - tRNA) is brought to the P - site of the small ribosomal subunit by eukaryotic initiation factor 2 (eIF2). It hydrolyzes GTP, and signals for the dissociation of several factors from the small ribosomal subunit, eventually leading to the association of the large subunit (or the 60S subunit). The complete ribosome (80S) then commences translation elongation . </P> <P> Regulation of protein synthesis is partly influenced by phosphorylation of eIF2 (via the α subunit), which is a part of the eIF2 - GTP - Met - tRNA ternary complex (eIF2 - TC). When large numbers of eIF2 are phosphorylated, protein synthesis is inhibited . This occurs under amino acid starvation or after viral infection . However, a small fraction of this initiation factor is naturally phosphorylated . Another regulator is 4EBP, which binds to the initiation factor eIF4E and inhibits its interactions with eIF4G, thus preventing cap - dependent initiation . To oppose the effects of 4EBP, growth factors phosphorylate 4EBP, reducing its affinity for eIF4E and permitting protein synthesis . </P> <P> While protein synthesis is globally regulated by modulating the expression of key initiation factors as well as the number of ribosomes, individual mRNAs can have different translation rates due to the presence of regulatory sequence elements . This has been shown to be important in a variety of settings including yeast meiosis and ethylene response in plants . In addition, recent work in yeast and humans suggest that evolutionary divergence in cis - regulatory sequences can impact translation regulation . Additionally, RNA helicases such as DHX29 and Ded1 / DDX3 may participate in the process of translation initiation, especially for mRNAs with structured 5'UTRs . </P> <P> The best - studied example of cap - independent translation initiation in eukaryotes is that by the Internal ribosome entry site (IRES). What differentiates cap - independent translation from cap - dependent translation is that cap - independent translation does not require the 5' cap to initiate scanning from the 5' end of the mRNA until the start codon . The ribosome can be trafficked to the start site by direct binding, initiation factors, and / or ITAFs (IRES trans - acting factors) bypassing the need to scan the entire 5' UTR . This method of translation has been found important in conditions that require the translation of specific mRNAs during cellular stress, when overall translation is reduced . Examples include factors responding to apoptosis and stress - induced responses . </P>

Describe the events in initiation and elongation of peptide chain in eukaryotes