LOCALIZATION OF PLASTID DNA-REPLICATION ON A NUCLEOID STRUCTURE

Plastids contain multiple copies of the plastid genome that are arrang ed into discrete aggregates, termed nucleoids. Nucleoid molecular orga nization and its possible role in ensuring genome continuity have not yet been carefully explored. We examined the relationship between plas tid DNA synthesis and nucleoid cytology in the unicellular chrysophyte Ochromonas danica, which is useful for such work because the genomes in each plastid are arranged in a single ring-shaped nucleoid. Immunoc ytochemical detection of thymidine analog incorporation into replicati ng DNA revealed that plastid DNA synthesis occurs at several sites alo ng the ring nucleoid simultaneously, and that all plastids of a single cell display similar replication patterns. Plastid DNA replication wa s observed in G(1), S, and G(2) phase cells. Pulse-chase-pulse labelli ng with two different thymidine analogs revealed that new sites are ac tivated as cells progress through the cell cycle while some old sites continue. The double labelling patterns suggest that the individual ge nomes are arranged consecutively, either singly or in clusters, along the nucleoid perimeter and that the selection of which genome replicat es when is a matter of chance. These observations eliminate a number o f alternative hypotheses concerning plastid DNA organization, and sugg est how cells might maintain a constancy of plastid DNA amount and why plastid genome variants segregate so rapidly during mitosis.