Mediates transposition of transposon Tn5 by a 'cut and paste' mechanism. First, the monomeric transposase binds the 19 bp inverted DNA repeats flanking the transposon. Then, dimerization of the DNA-bound transposase creates a synaptic DNA complex. After nicking of the first DNA strand, excision of the transposon proceeds through a series of intermediates. The transposase then mediates the insertion of the transposon at a new site by strand transfer. The activity of the wild-type transposase is very low, and is further inhibited by dimerization with the transposase inhibitor (inh). Binds 2 magnesium ions per subunit. Monomer. Homodimer of tnp (isoform 1), and heterodimer of tnp (isoform 1) and inh (isoform 2). Tn5 is a bacterial transposon that contains three genes conferring resistance to the antibiotics kanamycin/neomycin, bleomycin and streptomycin, flanked by two IS50 elements. IS50R codes for the functional transposase (isoform 1) and for the transposition inhibitor (isoform 2). IS50L differs by one nucleotide, giving rise to an ochre stop codon and producing C-terminally truncated proteins that lack transposase activity. Expressed at 4 times higher levels than isoform 1. Belongs to the transposase 11 family.