Vol. 9, Issue 7, 654-661, July 1999

LETTER
Separation of Micronuclear DNA of Stylonychia lemnae by Pulsed-Field Electrophoresis and Identification of a DNA Molecule with a High Copy Number

Institute for Cell Biology, University of Witten/Herdecke, D-58448 Witten, Germany

DNA from the hypotrichous ciliatae Stylonychia lemnae was separated by PFGE. In addition to the separation of the macronuclear DNA molecules with a size up to ~40 kb, we were able to separate the micronuclear DNA with a size between ~90 kb and 2 Mb. One very prominent 90-kb DNA band appeared on the pulsed-field gels. We propose that this 90-kb DNA fragment represents a linear plasmid residing in the micronucleus in a very high copy number. About 10% of the micronuclear DNA consists of the 90-kb DNA molecule. It appears in the micronucleus as well as in the macronuclear anlagen during macronuclear development but not in the mature macronucleus. Thus, the multicopy DNA is eliminated during fragmentation of the macronuclear anlagen DNA in the course of macronuclear development. Therefore, this 90-kb DNA molecule might serve as an excellent tool to study the recognition and elimination of DNA during nuclear differentiation of hypotrichous ciliates.