A New Generalizable Test for Detection of Mutations Affecting Tn10 Transposition

Olivier Huisman ¹ and Nancy Kleckner ²

¹ Department of Biochemistry, Harvard University, Cambridge, Massachusetts 02138
² Department of Molecular Biology, Harvard University, Cambridge, Massachusetts 02138

We describe here a new rapid screen that allows easy detection of transposon or host mutations that affect Tn10 transposition in Escherichia coli. This test involves a new Tn10 derivative called the "mini-lacZ-kanR fusion hopper" or mini-Tn10-LK for short. This element does not direct expression of ß-galactosidase when present at its original starting location on a suitably engineered plasmid or phage genome because it lacks appropriate transcription and translation start signals. However, transposition of this element into the chromosome of E. coli lacZ^- bacteria leads to productive fusions in which the lacZ gene within the transposon is expressed from external chromosomal signals. Such fusions are readily detectable on MacConkey lactose indicator plates as red (Lac⁺) papillae inside of white (LacZ^- ) colonies. The length of time required to see red papillae appearing in a white colony sensitively and accurately reflects the transposition frequency of the mini-transposon within the colonies. Differences in times for color formation are sensitive enough that 10-fold differences in transposition frequency can readily be detected. This papillation assay can be used to identify mutant clones in which the frequency of Tn10 transposition is either increased or decreased. We have successfully used the assay to identify mutations in the terminal sequences of Tn10; mutations in the Tn10 transposase gene or the bacterial host can be isolated just as easily. This screen should be readily adaptable to transposable elements other than Tn10.

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