The replicon fusion model for transposition predicts that each transposable element encodes a site-specific recombination activity (resolvase). A series of experiments were performed to test for the presence of such a resolvase in Tn5. None could be found when assays were devised to detect inter- or intramolecular recombination. An experiment was performed to determine if expression of resolvase occurs simultaneously with the transposition process, thus explaining the undetectable levels of site-specific recombination. In this case, no resolvase activity could be detected in cells that had undergone transposition. Instead, rearrangements were found that could be explained best by an alternative model of transposition. The rearrangements were consistent with a pathway that invokes excising the transposon from its initial site of insertion, followed by circularization of the element before its eventual transposition. The excision event may occur either with or without DNA synthesis.
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