May be involved in response to oxidative stress (By similarity). Toxic component of a type I toxin-antitoxin (TA) system. Neutralized by cognate antisense RNA antitoxin IsoA3 RNA which forms an extensive duplex with the mRNA. The antitoxin gene cannot be deleted, indicating expression of AapA3 is toxic. Non-cognate antitoxin RNAs IsoA1, IsoA2, IsoA4, IsoA5 or IsoA6 are unable to neutralize aapA3 (PubMed:31411564). Oxidative stress may trigger transformation of the bacteria to a coccoid form via the AapA type I TA systems. The peptide may penetrate the membrane leading to lipid reorganization and thinning of the bilayer (By similarity). Transcription of the aapA3 gene generates a full-length transcript whose folding impedes translation. Processing of the 3' end of the aapA3 message generates a shorter transcript that becomes translatable after a structural rearrangement. Expressed during exponential and stationary growth. Belongs to the AapA toxin family.