Nuclear rDNA in Euglena gracilis: paucity of chromosomal units and replication of extrachromosomal units.

Copy number of chromosomal rDNA units was investigated in two Euglena gracilis wild-type strains. It was established by dot blot analysis that these strains possess about four integrated units per haploid genome. This is the first example of a photosynthetic cell with only a few chromosomal ribosomal genes. In addition to these units, Euglena has 800 to 4000 extrachromosomal rDNA units. Electron microscopy revealed that these free rDNA circles bear a replication origin, and intermediates of replication show a D-loop structure.