IF3 licenses newly made 30S subunits for translation during stress

Bacterial ribosome biogenesis and translation occur in the same cellular compartment. Therefore, a biochemical gate-keeping step is required to prevent immature ribosomes from engaging in protein synthesis. Here, we show that the abundant ribosome assembly factor, RbfA, creates this checkpoint by suppressing protein synthesis by immature E. coli 30S subunits. After 30S maturation, RbfA is released by initiation factor 3 (IF3), which remains bound to 30S subunits to promote translation initiation. Genetic interactions between RbfA and IF3 show that IF3 is important for RbfA release during logarithmic growth. Moreover, IF3 is the main pathway for RbfA release in stationary phase when the activity of a less abundant RbfA-release factor, RsgA GTPase, is inhibited by the alarmone (p)ppGpp. By gating the transition from 30S biogenesis to translation initiation, RbfA and IF3 maintain the integrity of bacterial protein synthesis under a range of growth conditions and especially under stress.