An imbalance of HU synthesis induces mucoidy in Escherichia coli.

Mutations in a number of loci, including the lon gene, dramatically increase the production of colanic acid capsular polysaccharide and render Escherichia coli K-12 mucoid. The lon gene, which encodes an ATP-dependent protease, is localized at ten minutes on the E. coli map and is very closely linked to the hupB gene coding for one of the two subunits of the histone-like protein HU. Surprisingly the introduction of a multi-copy plasmid carrying either the hupB or hupA gene into a wild-type E. coli strain, results in the overproduction of one of the HU subunits and repression of the synthesis of the other without changing the overall concentration of HU, also renders the cells mucoid. As in a lon strain, the transcription of the cps genes, the structural genes for the synthesis of colanic acid, is induced dramatically. Protease Lon negatively regulates cps genes by destabilizing RcsA, a positive regulator of capsule synthesis. Regulation of HU synthesis does not affect the steady state level of Lon, as judged by Western blotting. The UV sensitivity of the hup transformed lon+ bacteria is identical to the lon+ parental strain, suggesting that Lon activity for the degradation of SulA in these cells is normal. Using lac operon fusions to cps gene promoters and to the rcsA promoter we show that the deregulation of HU synthesis does not by pass the positive regulatory action of RcsA and RcsB for the expression of cps genes but functions by stimulating RcsA synthesis.