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We recently demonstrated that both linear- and supercoil-form B1/B2 SINE (short interspersed elements) sequences could increase an integration frequency of a reporter gene in preimplatation mouse embryos. In those reports, when either a control or SINE-flanked DNA was separately applied to microinjection, the proportions of beta-gal positives were 16% and 63%, respectively, in linear-form DNA, and 6% and 25%, respectively, in circular-form DNA. Here, we examined the contribution of a circular-form DNA moiety to integration frequency by using a mixed-farm (linear and circular-form) DNA in microinjection. When examined in the blastocyst stage, the proportion of beta-gal-positive embryos was 17.3% and 46.6% in control and SINE-flanked DNA, respectively. These results suggest that there is little contribution of circular-form DNA moiety to the resultant integration frequency, and that the majority of the integration events are mediated through a linear conformation of vector DNA. In addition, some clues on integration process could be obtained from the analysis of microinjection results.
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