In E. coli, Ap4A has been shown to function as an alarmone, as the intracellular concentration of the molecule increases upon heat stress. Ap4A can also be incorporated into RNA as a 5' Cap along with other dinucleoside polyphosphates. It serves as a substrate for the RNA polymerase and the intracellular levels of these capped RNAs increase upon stress, suggesting that the cap adds a level of stability to the RNA.