γH2AX in the S Phase after UV Irradiation Corresponds to DNA Replication and Does Not Report on the Extent of DNA Damage

Ultraviolet (UV) radiation is a major environmental mutagen. Exposure to UV leads to a sharp peak of gamma H2AX, the phosphorylated form of the histone variant H2AX, in the S phase within an asynchronous population of cells. gamma H2AX is often considered a definitive marker of DNA damage inside a cell. In this report, we show that gamma H2AX in the S-phase cells after UV irradiation reports neither on the extent of primary DNA damage in the form of cyclobutane pyrimidine dimers nor on the extent of its secondary manifestations in the form of DNA double-strand breaks or in the inhibition of global transcription. Instead, gamma H2AX in the S phase corresponds to the sites of active replication at the time of UV irradiation. This accumulation of gamma H2AX at replication sites slows down the replication. However, the cells do complete the replication of their genomes and arrest within the G(2) phase. Our study suggests that it is not DNA damage, but the response elicited, which peaks in the S phase upon UV irradiation.