Apurinic/apyrimidinic (AP) sites are one of the most frequent DNA lesions. AP sites inhibit transcription and DNA
replication, and induce cell death. AP endonucleases are key enzymes in AP site repair. Several types of AP
endonucleases have been reported, such as AP endonuclease 2 (APEX2) and ribosomal protein P0 (P0). However, it is
not known how the functions and roles differ among AP endonucleases. To clarify the difference of roles among AP
endonucleases, we conducted biochemical analysis focused on APEX2 and P0 homologues in Ciona intestinalis. Amino
acid sequence analysis suggested that CiAPEX2 and CiP0 are AP endonuclease homologues. Although we could not
detect AP endonuclease or 3′-phosphodiesterase activity, these two purified proteins exhibited 3′-5′ exonuclease
activity. This 3′-5′ exonuclease activity was sensitive to ethylenediaminetetraacetic acid (EDTA), and the efficiency of this
activity was influenced by the 3′-terminus of substrate DNA. Both CiAPEX2 and CiP0 degraded not only a 5′-protruding
DNA end, but also nicked DNA, which is generated through AP endonuclease 1 (APEX1) cleavage. These two genes
partially complemented the growth rate of AP endonuclease-deficient Escherichia coli treated with hydrogen peroxide.
These results indicate that 3′-5′ exonuclease activity is an evolutionarily conserved enzymatic activity of APEX2 and P0
homologues and this enzymatic activity may be important for AP endonucleases.

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• CiAPEX2 and CiP0, candidates of AP endonucleases in Ciona intestinalis, have 3′-5′ exonuclease activity and contribute to protection against oxidative stress