The LysR-like regulator LeuO in Escherichia coli is involved in the translational regulation of rpoS by affecting the expression of the small regulatory DsrA-RNA.

The translation of rpoS, which encodes the general stress sigma factor, sigmaS, in Escherichia coli, is stimulated by various stress conditions. Regulatory factors involved in this control are the RNA-binding Hfq (HF-I) protein, the histone-like protein H-NS and the small regulatory DsrA-RNA (with the last being specifically required for increased rpoS translation at low temperature). Here, we report the characterization of a transposon insertion mutant (Tn10-8) with reduced sigmaS levels that led to the identification of an additional factor involved in the regulation of rpoS translation, the LysR-like regulator LeuO. Tn10-8 decreases rpoS translation predominantly at low growth temperature. The mutation results in similarly strongly reduced DsrA-RNA expression and does not affect rpoS expression in a dsrA null mutant background, indicating that it affects rpoS translation via DsrA-RNA. Tn10-8 is inserted 26bp upstream of the leuO open reading frame, which encodes a putative LysR-like regulator of unknown function. Instead of being a leuO null mutation, Tn10-8 activates leuO expression as a result of the p(out) promoter on IS10L reading into leuO, indicating that LeuO represses dsrA and thereby reduces rpoS translation at low temperature. LeuO does not contribute to temperature regulation of dsrA since its own expression is rather low and not temperature dependent. In a mutant deficient for H-NS, however, leuO is strongly derepressed. We conclude that rpoS translation is controlled by a regulatory network that includes Hfq, H-NS, LeuO and DsrA-RNA. In this network, H-NS plays a dual role by interfering with rpoS translation in general and, via LeuO, influencing the synthesis of its own low-temperature antagonist, DsrA-RNA.