DNA replication and transcription in new macronuclei of Paramecium caudatum exconjugants.

We analyzed the onset and location of replicational and transcriptional activity during the first cell cycle in new macronuclei of the ciliate Paramecium caudatum exconjugants. Synchronous exconjugants were pulse labeled with 5-bromo-2'-deoxyuridine or 5-bromouridine-5'-triphosphate to visualize replication or transcription sites, respectively. The first morphological change after macronuclear determination is the appearance of heterochromatic aggregates. Confocal microscopic examination revealed that DNA replication started at the stage when new macronuclear chromatin was partially decondensed, and that replication sites were located in a large number of small spot-like areas excluding the heterochromatic regions. Transcriptional activation in the new macronuclei also took place in the same developmental stage and in the same region that replication started. As macronuclear development progressed, heterochromatic aggregates disappeared, and replication and transcription sites were scattered throughout the nucleoplasm. Moreover, studies on aphidicolin-treated exconjugants demonstrated that inhibition of the DNA replication did not hinder transcriptional activation in the new macronuclei. On the other hand, replicational and transcriptional activity were also detected in old macronuclear fragments irrespective of their morphology and size, and length and timing of the replication corresponded to those in the new macronuclei.