AbstractThe effects of genotoxic agents on DNA structure, repair, and mutation—induction have shown many clear effects and mechanisms, yet their action on cellular transposons and insertion elements has been less well examined. We are interested in addressing the effect of environmental insults on the induction of DNA transposition of a cell's complement of mobile genetic elements. We have previously described a model system whereby we can measure the transposition potential of a cell by fusing bacteriophage Mu's (a plaque‐forming viral transposon) transposase (A gene) to that encoding β‐galactosidase (lacZ) and assaying for changes in β‐galactosidase expression after exposure of cells to toxic agents. We found that a known DNA‐damaging agent, γ‐rays, appeared to exert no discernible effect on Mu DNA transposition or transposase gene expression. In addition to monitoring the consequences of simple extrinsic agents, however, we were also curious to see how complex environmental mutagens influence DNA transposition and transposase gene expression.To accomplish this, we studied Mu transposase gene expression after exposure ofE.colicells harboring the MuA′‐′lacZ fusion plasmid pMD12 to six different cigarette smoke condensates (CSC). Upon exposure to CSC alone, no apparent pattern of changes in Mu transposase expression was observed in eitherrecA+orrecA−strains. However, when the CSC were treated with an Aroclor 1254 activated liver microsomal fraction, there was a consistent dose‐dependent decrease in transposase expression inrecA+E.coli, but not in an otherwise isogenicrecA−strain. This result is specific for Mu transposase expression as control strains, containing thelacZ gene under its own regulatory circuitry rather than that of the Mu transposase gene, did not exhibit a striking positive or negative effect on β‐galactosidase e