REGULATION OF REPLICON SIZE IN XENOPUS EGG EXTRACTS

Once a specific number of cells have been produced in the early Xenopu s laevis embryo, replicon size during the S phase of the cell cycle in creases. Here, it is reported that similar increase in replicon size o ccurred when the concentration of nuclei in replication-competent Xeno pus egg extracts exceeded a critical threshold. In this system, the or igin recognition complex (ORC) did not become stoichiometrically limit ing for initiation, and similar amounts of this complex bound to chrom atin regardless of replicon size. These data suggest that in early dev elopment, an unidentified factor controls how many preformed ORC-DNA c omplexes initiate DNA replication.