DNA replication arrest and tolerance to DNA methylation damage

Abstract

Inhibition of DNA replication by different DNA damaging agents has been investigated in HeLaMR cells and a methylation damage-tolerant variant HeLa5Al. In synchronous HeLaMR and HeLa5A1 cells exposed to N-ethyl-N-nitrosourea or ionizing radiation in mid-G₁ phase, DNA synthesis was 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 HeLa5Al cells affected neither the timing nor the extent of the first or second S phases. Both radiation and chemical treatment inhibited replication of an episomal plasmid and of genomic DNA in unison. Inhibition was observed at levels of DNA damage that did not directly damage the plasmid molecules. Thus, DNA replication inhibition occurs immediately after ionizing radiation or ethylation damage, but methylation damage requires processing through one cell cycle to generate an inhibitory signal. The inhibitory signal appears to act in trans on undamaged DNA. Although methylationtolerantcells are responsive to inhibition after λ-irradiation, methylation damage does not produce inhibitor signals to which they respond.