Control of transposon Tn5 transposition in Escherichia coli

Tn5 is a composite transposable element in which the insertion sequences IS50R and IS50L bracket a central region encoding kanamycin resistance (kanr). IS50R encodes a functional transposase, whereas IS50L contains the promoter of the kanr gene. To determine the relative activities of IS50R and IS50L in transposition we examined the structures of chimeric DNA molecules generated by insertion of segments of pBR322::Tn5 dimeric plasmids into red- lambda phage in recA- Escherichia coli. Restriction endonuclease analyses showed that the inserted sequences contained direct terminal repeats of pairs of IS50R or of IS50L elements and that the frequencies of usage of IS50R vs. IS50L depended on the position and orientation of Tn5 in the plasmid vector: IS50R was used preferentially when Tn5 was in transcriptionally quiescent regions of the vector (in either orientation) or when IS50L was immediately downstream from a strong promoter in the vector. In contrast, IS50L was used preferentially when IS50R was downstream from a strong promoter. We conclude IS50R tends to be used preferentially but that when transcription impinges on the end of an IS50 element the participation of that element in transposition is inhibited.