Structure of the replication fork in ultraviolet light-irradiated human cells

The DNA extracted from xeroderma pigmentosum human fibroblasts previously irradiated with 12.5 J/m2 of UV light and pulse-labeled for 45 min with radioactive and (or) heavy precursors, was used to determine the structural characteristics of the replication fork. Density equilibrium centrifugation experiments showed that a fork moved 6 micrometer in 45 min and bypassed 3 pyrimidine dimers in both strands. The same length was covered in 15-20 min in control cells. The delay in irradiated cells was apparently due to pyrimidine dimers acting as temporary blocks to the fork movement. Evidence for this interpretation comes from kinetics of incorporation of [3H]thymidine into DNA, which show that the time necessary to attain a new stable level of DNA synthesis in irradiated cells is equivalent to that required for the replication fork to cover the interdimer distance in one strand. On the other hand, the action of S1 nuclease on DNA synthesized soon after irradiation gives rise to a bimodal distribution in neutral sucrose gradients, one peak corresponding to 43 X 10(6) daltons and the other to 3 X 10(6) daltons. These two DNA species are generated by the attack of the S1 nuclease on single-stranded regions associated with the replication fork. A possible explanation for these results is given by a model according to which there is a delayed bypass of the dimer in the leading strand and the appearance of gaps opposite pyrimidine dimers in the lagging strand, as a direct consequence of the discontinuous mode of DNA replication. In terms of the model, the DNA of 43 X 10(6) daltons corresponds to the leading strand, linked to the unreplicated branch of the forks, whereas the piece of 3 X 10(6) daltons is the intergap DNA coming from the lagging strand. Pulse and chase experiments reveal that the low molecular weight DNA grows in a pattern that suggests that more than one gap may be formed per replication fork.