Attacins are antibacterial proteins synthesized by pupae of the giant silk moth, Hyalophora cecropia, in response to a bacterial infection. In this report we show that the previously described, attacin-induced alteration in the structure and the permeability of the outer membrane of Escherichia coli is associated with a specific inhibition of the synthesis of several outer membrane proteins, including OmpC, OmpF, OmpA, and LamB. The inhibition is expressed as a reduction in the steady-state mRNA levels and is at least in part the results of a block in transcription of the corresponding genes. Transcription directed by the promoter of ompR, the positive regulator of ompC and ompF expression in response to environmental conditions, is also affected by attacin. The effects on mutant strains show that the primary activity of attacin is not mediated by the ompR-envZ regulatory system. Instead our data suggest the existence in E. coli of a previously unknown system for the transcriptional regulation of a large set of outer membrane proteins previously not known to be coordinately regulated. We propose that the activity of attacin is directed towards this system.