Novel roles of LeuO in transcription regulation of E. coli genome: antagonistic interplay with the universal silencer H-NS.

LeuO, the regulator of leucine biosynthesis operon of Escherichia coli, is involved in the regulation of as yet unspecified genes affecting the stress response and pathogenesis expression. To get insights into the regulatory role(s) of LeuO, Genomic SELEX screening has been performed to identify the whole set of its regulation targets. A total of 140 LeuO-binding sites were identified on the E. coli genome, of which as many as 133 (95%) were found to contain the binding sites of H-NS, the universal silencer of stress-response genes, supporting the concept that LeuO plays an antagonistic role with anti-silencing activity. Western blot analysis indicated that H-NS predominates in growing phase; however, after prolonged culture for 1 week, H-NS decreased instead LeuO increased, supporting the anti-silencing role of LeuO. In concert with this model, a set of stress-response genes including cryptic chaperone/usher-type fimbriae operons are under the control of antagonistic interplay between LeuO and H-NS. Confocal laser scanning microscopic observation in flow-chambers showed that the mutants lacking leuO and some fimbriae genes are defective in biofilm formation or form altered biofilm architecture. Taken together we propose that LeuO is a major player in antagonistic interplay against the universal silencer H-NS.