EXCISION AND TRANSPOSITION OF TN5 AS AN SOS ACTIVITY IN ESCHERICHIA-COLI

Excision and transposition of the Tn5 element in Escherichia coli ordinarily appear to occur by recA-independent mechanisms. However, recA(Prt(c)) genes, which encode RecA proteins that are constitutively activated to the protease state, greatly enhanced excision and transposition; both events appeared to occur concomitantly and without destruction of the donor DNA. The recombinase function of the RecA protein was not required. Transposition was accompanied by partial, and occasionally full, restoration of the functional integrity of the gene vacated by the excised Tn5. The stimulation of transposition was inhibited by an uncleavable LexA protein and was strongly enhanced by an additional role of the RecA(Prt(c)) protein besides its mediation of LexA cleavage. To account for the enhanced transposition, we suggest that (i) there may be a LexA binding site within the promoter for the IS50 transposae, (ii) activated RecA may cleave the IS50 transposition inhibitor, and (iii) the transposase may be formed by RecA cleavage of a precursor molecule.