Extrachromosomal unequal homologous recombination and gene conversion in simian kidney cells: effects of UV damage

Abstract

Shuttle plasmid vectors containing the SV40 origin of replication and tandem neo genes with distally placed non-overlapping deletions were used to study the effects of DNA damage on extrachromosomal homologous recombination in simian kidney cells. DNA was introduced into COS7 cells by a lipofectin-mediated transfection procedure and recombination was assessed by analyzing the structure of plasmids. Recombinational events observed included unequal homologous recombination (triplication), gene conversion, double reciprocal recombination, deletion (pop-outs), gene amplification (4–6 copies), and multimerization. Triplication, an event that previously had not been reported in association with extrachromosomal recombination, predominated in experiments with undamaged vectors. The recombination frequency (NeoR/AmpR) of vectors randomly damaged by UV irradiation was essentially unchanged; however, the relative number of triplication events decreased significantly. Selective damage in one of the two neo genes increased the relative frequency of gene conversion. The experimental system developed for use in this study detects all major homologous recombination events observed in chromosomal direct repeat sequences in mammalian cells and yeast and should prove valuable for future studies of homologous recombination in mammalian cells.