Spatiotemporal sites of DNA replication in macro- and micronuclei of the ciliate Paramecium caudatum.

Spatiotemporal sites of DNA replication in macro- and micronuclei of the ciliated protozoan Paramecium caudatum were analyzed by confocal laser scanning microscopy following incorporation of the thymidine analogue BrdU and indirect immunofluorescence. In the macronucleus, replication sites were localized to numerous small domains and scattered throughout the nucleoplasm. This pattern persisted during all periods of the S phase. A single constant pattern with discrete replication foci was also observed in the micronucleus. No obvious differences were seen between the two kinds of nuclei. Pulse-chase-pulse double-labeling experiments with two thymidine analogues (CldU and IdU) revealed that dispersed sites of replication were activated at different times during the S phase and a replication site takes about 2h to complete replication in the macronucleus. When cells were labeled by BrUTP to examine transcriptional activity in the two kinds of nuclei, incorporation of BrUTP into the macronucleus occurred throughout the cell cycle, whereas there was no detectable RNA synthesis in the micronucleus. From these findings, we conclude that, despite large differences in structure and function of macro- and micronuclear genomes, both nuclei show a similar replication pattern with discrete subnuclear foci scattered throughout the nucleoplasm at all times during the S phase.