The leuO Gene Product Has a Latent Ability To Relieve bgl Silencing in Escherichia coli

The Escherichia coli bgl operon is of interest, since its expression is silent (phenotypically Bgl⁻), at least under standard laboratory conditions. Here we attempted to identify a trans-acting factor(s) that is presumably relevant to the regulation of bgl by a random insertion mutagenesis with mini-Tn10. These collected mutations, conferring the phenotype of Bgl⁺, were localized in three loci on the genetic map, two of which appeared to be hns and bglJ, which were previously implicated as the factors affecting the Bgl phenotype. The other locus at 1 to 2 min on the genetic map appeared to be a new one. In this case, the insertion mutation was found to be just in front of the leuO gene encoding a putative LysR-like DNA-binding protein. Genetic analyses revealed that overproduction of LeuO in the wild-type cells causes the phenotype of Bgl⁺. A leuO deletion mutant was also characterized in terms of expression of bgl. From these results, the possible function of LeuO in bgl expression will be discussed from an evolutionary and/or ecological point of view.