Control of DNA Replication by the Nucleus/Cytoplasm Ratio in Xenopus

The nucleus/cytoplasm (N/C) ratio controls S phase dynamics in many biological systems, most notably the abrupt remodeling of the cell cycle that occurs at the midblastula transition in early Xenopus laevis embryos. After an initial series of rapid cleavage cycles consisting only of S and M phases, a critical N/C ratio is reached, which causes a sharp increase in the length of S phase as the cell cycle is reconfigured to resemble somatic cell cycles. How the N/C ratio determines the length of S phase has been a longstanding problem in developmental biology. Using Xenopus egg extracts, we show that DNA replication at high N/C ratio is restricted by one or more limiting substances. We report here that the protein phosphatase PP2A, in conjunction with its B55α regulatory subunit, becomes limiting for replication origin firing at high N/C ratio, and this in turn leads to reduced origin activation and an increase in the time required to complete S phase. Increasing the levels of PP2A catalytic subunit or B55α experimentally restores rapid DNA synthesis at high N/C ratio. Inversely, reduction of PP2A or B55α levels sharply extends S phase even in low N/C extracts. These results identify PP2A-B55α as a link between DNA replication and N/C ratio in egg extracts and suggest a mechanism that may influence the onset of the midblastula transition in vivo . Background: The duration of S phase in Xenopus is controlled by the nucleus/cytoplasm ratio through unknown mechanisms. Results: Modulation of protein phosphatase 2A (PP2A) levels controls origin firing at high nucleus/cytoplasm. Conclusion: Depletion of PP2A and its B55α subunit at high nucleus/cytoplasm causes an extension of S phase. Significance: This mechanism may influence cell cycle remodeling that occurs in early embryogenesis.