A p53-dependent S-phase checkpoint helps to protect cells from DNA damage in response to starvation for pyrimidine nucleotides

Normal mammalian cells arrest primarily in G(1) in response to N-(phosphona cetyl)-L-aspartate (PALA), which starves them for pyrimidine nucleotides, a nd do not generate or tolerate amplification of the CAD gene, which confers resistance to PALA. Loss of p53, accompanied by loss of G(1) arrest, permi ts CAD gene amplification and the consequent formation of PALA-resistant co lonies. We have found rat and human cell lines that retain wild-type p53 bu t have lost the ability to arrest in G(1) in response to PALA. However, the se cells still fail to give PALA-resistant colonies and are protected from DNA damage through the operation of a second checkpoint that arrests them r eversibly within S-phase. This S-phase arrest, unmasked in the absence of t he G1 checkpoint, is dependent on p53 and independent of p21/waf1.