A bacterial initiation factor (IF) is a protein that stabilizes the initiation complex for polypeptide translation.
Translation initiation is essential to protein synthesis and regulates mRNA translation fidelity and efficiency in bacteria. The 30S ribosomal subunit, initiator tRNA, and mRNA form an initiation complex for elongation. This complex process requires three essential protein factors in bacteria – IF1, IF2, and IF3. These factors bind to the 30S subunit and promote correct initiation codon selection on the mRNA. IF1, the smallest factor at 8.2 kDa, blocks elongator tRNA binding at the A-site. IF2 is the major component that transports initiator tRNA to the P-site. IF3 checks P-site codon-anticodon pairing and rejects incorrect initiation complexes.
The orderly mechanism of initiation starts with IF3 attaching to the 30S subunit and changing its shape. IF1 joins next, followed by mRNA binding, and starts codon-P-site interaction. IF2 enters with the initiator tRNA and places it on the start codon. GTP hydrolysis by IF2 releases it and IF3, enabling 50S subunit joining. The coordinated binding and activities of IF1, IF2, and IF3 are essential for the rapid and precise translation initiation in bacteria. They facilitate start codon selection and assemble an active, protein-synthesis-ready 70S ribosome.