(p)ppGpp – small molecule – major impact

The alarmone (p)ppGpp has a key regulatory role in coordinating bacterial cellular response in order to adapt to environmental changes. Over the years (p)ppGpp was also reported to be involved in many other survival pathways such as: resistance to beta-lactam and peptide antibiotics, formation of persister cells, sporulation and biofilm formation. Bacterial persistence has been shown to be an underlying factor in the failure of antibiotic treatments. The onset of the stringent response seems to trigger persister formation. It was found that upon expression of the first toxin linked to persister formation HipA, or upon degradation of its antitoxin HipB, GltX (glutamyl-tRNA-synthetase) undergoes phosphorylation at Ser239, its ATP-binding site. This phosphorylation leads to the accumulation of uncharged tRNA^Glu in the cell, which results in the activation of the stringent response. Other survival mechanisms, such as entry to the stationary phase, sporulation and biofilm formation, are also controlled by the accumulation of (p)ppGpp. Based on structural studies, analogs which inhibit RelA activity both in vivo and in vitro were constructed. The analogs inhibit both RelA from Gram negative and Rel/Spo from Gram positive bacteria. Using these analogs and inhibiting (p)ppGpp synthesis, Bacillus subtilis cells were not able to enter the stationary phase which resulted in cell death. (p)ppGpp analogues were also able to prevent cells from undergoing sporulation and form intact biofilm, which both resulted in bacterial cell death.