 Bacterial translation initiation factor

A bacterial translation initiation factor is a protein that helps assemble and select a productive ribosomal initiation state. The canonical factors are bacterial initiation factor IF1, bacterial initiation factor IF2 and bacterial initiation factor IF3. Their cooperation influences recruitment, retention and timing of components on the 30S ribosomal subunit rather than simply increasing an undifferentiated amount of RNA binding.^[ref]

A real-time assembly study identified a kinetically favored pathway in which IF2 and IF3 associate before IF1 stabilizes the assembly. Messenger RNA recruitment need not wait for a single obligatory point in that pathway; its behavior also depends on concentration and message structure.^[ref]

An original two-state model shows why equilibrium occupancy is not the whole story. Let a site switch from unbound to bound at a pseudo-first-order rate $a$ and back at rate $b$. The bound fraction at equilibrium iswhere $\tau$ is the relaxation time of this stipulated model. Both $a=b=1{\mathrm{s}}^{-1}$ and $a=b=10{\mathrm{s}}^{-1}$ give 50-percent occupancy, but their relaxation times are 0.50 and 0.05 seconds. A factor that changes rates can therefore alter assembly timing without producing a corresponding change in final occupancy.

Real initiation involves more states than this model. Distinguishing a stable but nonproductive assembly from a productive 30S initiation complex requires an appropriate functional or positional readout, not binding measurements alone.