Separation of micronuclear DNA of Stylonychia lemnae by pulsed-field electrophoresis and identification of a DNA molecule with a high copy number

DNA from the hypotrichous ciliatae Stylonychia lemnae was separated by PFGE . In addition to the separation of the macronuclear DNA molecules with a si ze up to similar to 40 kb, we were able to separate the micronuclear DNA wi th a size between similar to 90 kb and 2 Mb. One very prominent 90-kb DNA b and appeared on the pulsed-field gels. We propose that this 90-kb DNA Fragm ent 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 m olecule. It appears in the micronucleus as well as in the macronuclear anla gen during macronuclear development but not in the mature macronucleus. Thu s, 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 a nd elimination of DNA during nuclear differentiation of hypotrichous ciliat es.