Activation of alternative sites of replicon initiation in Chinese hamster cells exposed to ultraviolet light

Exposure to UV light is known to produce lesions that block DNA polymerases at least on the leading strand. If several lesions are present in adjacent replicons, it is likely that sections of DNA would remain unreplicated because of the presence for blocking lesions. For cells to multiply and survive these areas must eventually be replicated. One mechanism that has been postulated to be involved in the replication of DNA between two blocking lesions is the activation of alternative sites of replicon initiation.

To detect the existence of alternative sites of replicon initiation we employed the high specific/low specific activity labelling protocol first used by Huberman and Riggs (1968) for DNA fiber autoradiography. After development of the autoradiographs, the distances between adjacent sites of replicon initiation (inter-origin distances) were measured. In both wild-type Chinese hamster ovary (CHO) cells and UV-5 CHO cells, which exhibit no excision repair abilities, the inter-origin distances were, on average, shorter in cells exposed to UV, indicating that exposure to UV results in the activation of alternative sites of initiation. This activation appears to occur immediately after UV in both cell lines, but persist for a longer time in the excision-deficient line.