Starvation-independent alarmone production inhibits translation through GTP depletion

Bacteria produce the alarmone nucleotides (p)ppGpp during stress to affect replication, transcription, translation, and metabolism. Recently, pGpp was identified as a third alarmone that is produced from the hydrolysis of (p)ppGpp. Although pGpp is a major component of bacterial stress responses, its precise role in mediating these responses is poorly understood. ppGpp and pppGpp bind translation GTPases and therefore directly affect translation to conserve resources during periods of stress. Here, we show that while pGpp is a weaker inhibitor of protein synthesis than ppGpp and pppGpp in vitro, pGpp production in the model Gram-positive bacterium Bacillus subtilis leads to faster translation inhibition in vivo. Faster translation inhibition is accompanied by greater levels of disengaged ribosomal subunits and hibernating ribosome dimers, suggesting that translation initiation is strongly inhibited. We show that alarmone production in vivo causes a severe depletion of GTP, which is sufficient for translation inhibition. Finally, we find that pGpp production also causes more robust transcriptome remodeling than (p)ppGpp production. This work supports a model that implicates all three alarmones in translation inhibition via GTP depletion.