Cell cycle modulation of protein-DNA interactions at a human replication origin.

We followed the variations of protein-DNA interactions occurring in vivo over the early firing replication origin located near the human lamin B2 gene, in IMR-90 cells synchronized in different moments of the cell cycle. In G0 phase cells no protection is present; as the cells progress in G1 phase an extended footprint covering over 100 bp appears, particularly marked at the G1/S border. As the cells enter S phase the protection shrinks to 70 bp and remains unchanged throughout this phase. In mitosis the protection totally disappears, only to reappear in its extended form as the cells move into the next G1. These variations are reminiscent of those corresponding to the formation of the pre- and post-replicative complexes described in yeast and Xenopus cells.