Induction of specific macronuclear developmental mutations by microinjection of a cloned telomeric gene in Paramecium primaurelia.

In Paramecium, the differentiation of a highly polyploid macronucleus from a diploid nucleus is accompanied by an extensive reorganization of the genome, involving reduction in chromosome size and formation of new telomeres at heterogeneous, but reproducible, positions. The results presented here, as well as work by others, indicate that telomere addition regions are not strictly determined by the micronuclear sequence, but are at least partially controlled by the old macronucleus. It is shown that microinjecting a high copy number of a plasmid containing the G surface antigen gene into the macronucleus of wild-type cells specifically modifies the processing of the G gene-bearing micronuclear chromosome at the following autogamy. Telomeric repeats are added upstream of the gene, rather than at their wild-type position 5 kb downstream of its 3' end, resulting in the deletion of the gene from the new macronucleus. This macronuclear mutation is unstable at the following autogamy, giving rise to many different telomere addition regions in different postautogamous clones. However, after several successive autogamies, cell lines can be obtained in which the telomeres reproducibly form in the same region. In crosses with wild-type cells, these macronuclear mutations show cytoplasmic inheritance; the micronuclei of the mutants are shown to be fully functional. The implications for the mechanism of choice of telomere addition sites are discussed.