Diadenosine 5′, 5′′′-P1,P4-tetraphosphate (Ap4A) is synthesized in response to DNA damage and inhibits the initiation of DNA replication.

Abstract The level of intracellular diadenosine 5′, 5′′′- P 1 , P 4 -tetraphosphate (Ap 4 A) increases several fold in mammalian cells treated with non-cytotoxic doses of interstrand DNA-crosslinking agents such as mitomycin C. It is also increased in cells lacking DNA repair proteins including XRCC1, PARP1, APTX and FANCG, while >50-fold increases (up to around 25 μM) are achieved in repair mutants exposed to mitomycin C. Part of this induced Ap 4 A is converted into novel derivatives, identified as mono- and di-ADP-ribosylated Ap 4 A. Gene knockout experiments suggest that DNA ligase III is primarily responsible for the synthesis of damage-induced Ap 4 A and that PARP1 and PARP2 can both catalyze its ADP-ribosylation. Degradative proteins such as aprataxin may also contribute to the increase. Using a cell-free replication system, Ap 4 A was found to cause a marked inhibition of the initiation of DNA replicons, while elongation was unaffected. Maximum inhibition of 70–80% was achieved with 20 μM Ap 4 A. Ap 3 A, Ap 5 A, Gp 4 G and ADP-ribosylated Ap 4 A were without effect. It is proposed that Ap 4 A acts as an important inducible ligand in the DNA damage response to prevent the replication of damaged DNA.