Mutations in the repressor encoded by the transposon Tn3 tnpR gene lead to increased levels of expression of two gene products: the mutant repressor (TnpR-) and the Tn3 encoded transposase, TnpA (Heffron et al. 1978; Chou et al. 1979a). Derivatives of the ColE1::Tn3 composite plasmid, RSF2124, with mutant Tn3 repressor exhibited the expected elevated levels of transposition. Unexpectedly, hosts containing these tnpR- derivatives produced enhanced levels of the ColE1 encoded toxin, colicin E1. The gene for colicin E1 maps far (0.23-0.98 MU) from the Tn3 insertion point (0.73 MU) (Fig. 1). The colicin E1 overproduction phenotype, designated Eop-, was complemented in trans by wild type repressor gene product (TnpR+) to the wild type phenotype, Eop+. Hosts with RSF2124 derivatives which expressed high levels of both mutant repressor and mutant transposase (TnpR-, TnpA-) were Eop-. Hosts containing plasmids deleted for both tnpA and tnpR promoters were Eop+, while hosts with plasmids carrying a lac promoter substitution for the tnpA promoter were Eop-. These data support the idea that a cis-acting effect of increased transcription from the tnpA promoter into adjacent ColE1 DNA was the cause of colicin overproduction. Increased transcription activated a putative colicin augmentation function (caf) whose presence was required for the Eop- phenotype. Deletion mapping established that one boundary of the caf locus lies within 52 bases of the junction of the left end of Tn3 and ColE1 DNA. ColE1 DNA in this area contains an open reading frame which could encode either a 74 or a 63 residue protein (B. Polisky, unpublished DNA sequence data). The presence of increased levels of an mRNA transcript from this region and/or the increased expression of protein(s) from this transcript could result in an Eop- phenotype. Expression of the Eop- phenotype requires the presence of the host recE gene. Evidence is presented which suggests that the recA repressor, lexA protein, controls expression of the recE gene product, ExoVIII.