The efficiency and timing of plasmid DNA replication in Xenopus eggs: correlations to the extent of prior chromatin assembly

Injection of the circular plasmid FV1 (derived from type I bovine papilloma virus) into Xenopus eggs before the start of the first cell cycle dramatically increases the efficiency of plasmid replication once eggs are chemically activated. We call this the preloading effect and report kinetic and quantitative characterization of this phenomenon here. The timing and the amount of FV1 synthesis were measured by both BrdUTP density labelling and an optimized method of selective enzymatic digestion of replicated and unreplicated molecules using the three methyladenosine-sensitive isoschizomers, DpnI, MboI and Sau3a. DpnI in 100 mM NaCl proved particularly useful for distinguishing and quantitating unreplicated, once-replicated, and repeatedly replicated molecules accumulated over several cell cycles. Our results reveal that both the amount of DNA replicated and the timing of synthesis during the first S-phase correlate with the length of the preloading period. Longer preloading leads to larger amounts of DNA being replicated sooner. In fact, up to 30–50% of 1 ng injected plasmid can replicate in a semiconservative cell cycle-dependent manner during the first S-phase. But such high levels of synthesis during the first cell cycle appear to limit the egg's ability to rereplicate this material in subsequent cell cycles. The preloading effect does not depend on synthesis of either viral or egg proteins, but does appear to correlate with the extent of plasmid assembly into chromatin before the start of the cell cycle. We postulate that each plasmid molecule must achieve a critical degree of chromatin assembly before it can proceed along the replication pathway. These observations illuminate some of the difficulties inherent in building a vector for gene insertion into Xenopus embryos, but also suggest an experimental strategy toward this aim.