Enhanced DNA strand exchange on positively charged liposomes

Abstract

The mechanism of DNA strand exchange, performed in vivo by proteins in the RecA family, is, despite extensive studies, still not understood in detail. We are therefore investigating a model system, using the surface of a lipid vesicle, to systematically study the effect of individual parameters on the rate of the strand exchange reaction. DNA oligonucleotides are accumulated on the surface of cationic liposomes and the strand exchange is monitored using FRET. We show that in the presence of lipid vesicles containing 35% cationic lipid the strand exchange takes place with a significantly enhanced rate and improved yield, compared to in bulk without liposomes. The highest rate is observed at a charge ratio where the positive surface charge is matched by the negative charges from DNA, and thus all double- and single-stranded DNA can be bound to the liposome surface. The DNA duplex is destabilized on the positively charged surface and with the accumulation of complementary single strands in close proximity a fast exchange is facilitated. We also show that when aggregation of liposomes is minimized by incorporation of a PEG-modified lipid, the strand exchange is still efficient, although with a lower rate than for the unmodified system. We conclude that the liposome model system, with the possibility to systematically vary the surface composition, is a promising platform for further studies of DNA strand exchange reactions.