DNA-REPLICATION ARREST AND TOLERANCE TO DNA METHYLATION DAMAGE

Inhibition of DNA replication by different DNA damaging agents has bee n investigated in HeLaMR cells and a methylation damage-tolerant varia nt HeLa5A1. In synchronous HeLaMR and HeLa5A1 cells exposed to N-ethyl -N-nitrosourea or ionizing radiation in mid-G(1) phase, DNA synthesis, vas inhibited in the following S phase. N-methyl-N-nitrosourea-induced replication inhibition in HeLaMR cells was delayed until the second S phase after treatment. In contrast, N-methyl-N-nitrosourea treatment of HeLa5A1 cells affected neither the timing nor the extent of the fir st or second S phases. Both radiation and chemical treatment inhibited replication of an episomal plasmid and of genomic DNA in unison. Inhi bition was observed at levels of DNA damage that did not directly dama ge the plasmid molecules. Thus, DNA replication inhibition occurs imme diately after ionizing radiation or ethylation damage, but methylation damage requires processing through one cell cycle to generate an inhi bitory signal. The inhibitory signal appears to act in trans on undama ged DNA. Although methylation-tolerant cells are responsive to inhibit ion after gamma-irradiation, methylation damage does not produce inhib itory signals to which they respond.