Antisense PNA effects in Escherichia coli are limited by the outer-membrane LPS layer

Antisense peptide nucleic acids (PNAs) can inhibit Escherichia coli gene ex pression and cell growth through sequence-specific RNA binding, and this op ens possibilities for novel anti-infective agents and tools for microbial f unctional genomics. However, the cellular effects of PNAs are limited relat ive to effects in cell extracts, presumably because of cell barrier compone nts such as the outer-membrane lipopolysaccharide (LPS) layer or drug efflu x pumps, both of which function to exclude antibiotics and other foreign mo lecules. To evaluate the importance of such cellular factors on RNA effects , the authors developed a positive assay for antisense inhibition by target ing the lac operon repressor and compared RNA susceptibilities in mutant an d wild-type E. coli by assessing lacZ induction. Strains with defective LPS (AS19 and D22) were more permeable to the antibiotic nitrocefin and more s usceptible to RNA than the wild-type. Also, RNA potency was improved in wil d-type cells grown in the presence of certain cell-wall-permeabilizing agen ts. In contrast, the activities of the Acr and Emr drug efflux pumps were n ot found to affect RNA susceptibility. The results show that the LPS layer is a major barrier against cell entry, but PNAs that can enter E. coli are likely to remain active inside cells.