Studies on the integration of RmI into the genome of rat cells
Wallenburg, John C.
RmI is a naturally occurring, circular, recombinant molecule consisting of a complete polyoma (Py) genome with an insertion in its late region of 1.6 kbp of mouse cellular DNA (Ins). The Py genome carries a mutation which renders its replication but not its transforming ability, température sensitive. Rml is produced upon induction of a specific clone of Py transformed mouse cells with the Ins sequences originating from the cellular sequences flanking the integrated Py genome on one side. Ins contains both single copy mouse cellular sequences and repetitive cellular sequences common to mouse, rat and hamster cells. The two viral-cellular junctions consist of a direct repeat of Py sequences of 182 bp and an imperfect inverted repeat of 7 bp of cellular DNA. Because of its particular structure and composition, Rml has the potentiel to integrated via illegitimate, homologous and possibly site-specific recombination. I transfected rat cells with Rml at the permissive and non-permissive temperature for replication, isolated 79 clones which had integrated Rml on the basis of their ability to form foci, and determined the detailed intégration patterns for 37 of these clones. The results led to several conclusions: (1) The presence of clones with identical digestion patterns suggests that there may be certain regions on the host cellular and Rml genomes that are more receptive as integration sites than others, (2) depending on the assay conditions used (restrictive or non-restrictive) the same donor molecule (Rml) produces a random or non-random integration pattern and (3) the difference between the integration patterns probably reflects a difference between the states of the integrating substrates. (4) the non-random pattern involves the cellular sequences in Ins, suggesting homologous interactions between them and their cellular counterparts. Finally, when both mechanisms are active then the recombination mechanism producing a random integration pattern is the preferred pathway for integrating Rml.