Tn4652 is a derivative of the toluene degradation transposon Tn4651 that belongs to the Tn3family of transposons (M. Tsuda and T. Iino, Mol. Gen. Genet. 210:270–276, 1987). We have sequenced the transposase genetnpA of transposon Tn4652 and mapped its promoter to the right end of the element. The deduced amino acid sequence of tnpA revealed 96.2% identity with the putative transposase of Tn5041. Homology with other Tn3family transposases was only moderate (about 20 to 24% identity), suggesting that Tn4652 and Tn5041 are distantly related members of the Tn3 family. Functional analysis of the tnpA promoter revealed that it is active inPseudomonas putida but silent in Escherichia coli, indicating that some P. putida-specific factor is required for the transcription from this promoter. Additionally,tnpA promoter activity was shown to be modulated by integration host factor (IHF). The presence of an IHF-binding site upstream of the tnpA promoter enhanced the promoter activity. The positive role of IHF was also confirmed by the finding that the enhancing effect of IHF was not detected in the P. putida ihfA-deficient strain A8759. Moreover, the Tn4652 terminal sequences had a negative effect on transcription from the tnpA promoter in theihfA-defective strain. This finding suggests that IHF not only enhances transcription from the tnpA promoter but also alleviates the negative effect of terminal sequences of Tn4652 on the promoter activity. Also, an in vitro binding assay demonstrated that both ends of Tn4652 bind IHF from a cell lysate of E. coli.