The protease Lon and the RNA-binding protein Hfq reduce silencing of the Escherichia coli bgl operon by H-NS.

The histone-like nucleoid structuring protein H-NS represses the Escherichia coli bgl operon at two levels. H-NS binds upstream of the promoter, represses transcription initiation, and binds downstream within the coding region of the first gene, where it induces polarity of transcription elongation. In hns mutants, silencing of the bgl operon is completely relieved. Various screens for mutants in which silencing of bgl is reduced have yielded mutations in hns and in genes encoding the transcription factors LeuO and BglJ. In order to identify additional factors that regulate bgl, we performed a transposon mutagenesis screen for mutants in which silencing of the operon is strengthened. This screen yielded mutants with mutations in cyaA, hfq, lon, and pgi, encoding adenylate cyclase, RNA-binding protein Hfq, protease Lon, and phosphoglucose isomerase, respectively. In cyaA mutants, the cyclic AMP receptor protein-dependent promoter is presumably inactive. The specific effect of the pgi mutants on bgl is low. Interestingly, in the hfq and lon mutants, the downstream silencing of bgl by H-NS (i.e., the induction of polarity) is more efficient, while the silencing of the promoter by H-NS is unaffected. Furthermore, in an hns mutant, Hfq has no significant effect and the effect of Lon is reduced. These data provide evidence that the specific repression by H-NS can (directly or indirectly) be modulated and controlled by other pleiotropic regulators.