Final answer:
The factor that promotes the binding of the first tRNA to the mRNA in the formation of the initiation complex is the eukaryotic initiation factor-2 (eIF-2) in eukaryotes and initiation factor-2 (IF-2) in prokaryotes.
Step-by-step explanation:
In the formation of the initiation complex during the process of protein synthesis, the binding of the first tRNA to the mRNA is promoted by the factor called eukaryotic initiation factor-2 (eIF-2). In eukaryotes, the initiator methionine tRNA (tRNAi^Met) is brought to the 40S ribosome by eIF-2 that is bound to GTP, forming an initiation complex. For prokaryotes like E. coli, initiation factor-2 (IF-2) performs a similar role, delivering the formylmethionine tRNA (fMet-tRNAfMet) to the small 30S ribosome subunit.
The process is controlled by the presence of GTP and involves other initiation factors that recognize the mRNA's 5' cap and poly-A tail. The assembly of the translation initiation complex is required for the ribosome to begin synthesizing a polypeptide chain.