We report the construction and application of a novel insertion element for transposase-mediated mutagenesis in gram-negative bacteria. Besides Km(r) as a selectable marker, the insertion element InsTet(G-)1 carries the anhydrotetracycline (atc)-regulated outward-directed PA promoter so that atc-dependent conditional gene knockouts or knockdowns are generated. The complex formed between the purified hyperactive transposase and InsTet(G-)1 was electroporated into Escherichia coli or Salmonella enterica serovar Typhimurium, and mutant pools were collected. We used E. coli strains with either TetR or the reverse variant revTetR(r2), while only TetR was employed in Salmonella. Screening of the InsTet(G-)1 insertion mutant pools revealed 15 atc-regulatable auxotrophic mutants for E. coli and 4 atc-regulatable auxotrophic mutants for Salmonella. We have also screened one Salmonella mutant pool in murine macrophage-like J774-A.1 cells using ampicillin enrichment. Two mutants with the InsTet(G-)1 insertion in the gene pyrE or argA survived this procedure, indicating a reduced intracellular growth rate in J774-A.1 cells. The nature of the mutants and the modes of their regulation are discussed. DOI: 10.1128/AEM.00492-06