Prokaryotic initiation factors

Prokaryotes require the use of three initiation factors: IF-1, IF-2, and IF-3.

IF-1

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The prokaryotic factor IF-1 associates with the 30S ribosomal subunit in the A site and prevents aminoacyl-tRNA from entering. It may also prevent the 50S subunit from binding, stopping the formation of the 70S subunit.

IF-2

IF-2 binds to an initator tRNA and controls the entry of that tRNA into the ribosome. IF-2, bound to GTP, binds to the 30S P site. After associating with the 30S subunit, fMet-tRNA_f binds to the IF-2 then IF-2 transfers the tRNA ino the partial P site. When the 50S subunit joins, it causes GTP to form GDP, causing a conformational change in the IF-2 that causes IF-2 to release and allow the 70S subunit to form.

IF-3

IF-3 is required for the 30S subunit to bind to the initiation site in mRNA. In addition, it has several other jobs including the stabilization of free 30S subunits, enables 30S subunits to bind to mRNA and checks for accuracy against the first aminoacyl-tRNA. IF-3 is required by the small subunit to form initiation complexes, but has to be released to allow the 50S subunit to bind.