Inhibition of SV40 Replication In Vitro by Poly(ADP-Ribosyl)ated Diadenosine Tetraphosphate

Abstract

Although poly(ADP-ribosyl)ation is generally considered as a post- translational modification of protein, poly(ADP-ribose) polymerase may also catalyze the addition of poly(ADP-ribose) to the 5′ dinucleotide diadenosine-5′,5′″-P1, P4-tetraphosphate (Ap4A) (1). 5′-Dinucleotides such as Ap4A have been identified in a number of biological systems, including bacteria, plants, and mammalian cells, yet the physiological role of these compounds remain unclear (reviewed in 2 and 3). One proposed role for Ap4A and related compounds is as “alarmones” or mediators of the cellular stress response (4, 5). This proposal is based on the observation that Ap4A accumulates in cells following a number of physiological stresses, including heat shock and oxidative stress (4–8). Ap4A has also been proposed to serve as a positive regulator of DNA replication (9, 10). Support for this model include reports of 50–1000 fold increases in the Ap4A pools of cells in S phase (9, 10) and that Ap4A initiates DNA synthesis in permeablized growth arrested cells (11). It should be noted, however, that these effects have not been observed by all investigators (5, 6, 12, 13). Ap4A could regulate either the cellular stress response or the initiation of DNA replication through an Ap4A binding protein that has been identified as part of the 640 kDa DNA polymerase alpha holoenzyme (EC 2.7.7.7) (14). Although neither the presence of this protein nor Ap4A seem to alter the catalytic activity of DNA polymerase alpha in vitro (15), the presence of the Ap4A binding protein allows Ap4A to serve as a primer for DNA synthesis in the absence of an RNA primer (16).