Overproduction of exopolysaccharides by an Escherichia coli K-12 rpoS mutant in response to osmotic stress.

The yjbEFGH operon is implicated in the production of an exopolysaccharide of an unknown function and is induced by osmotic stress and negatively regulated by the general stress response sigma factor RpoS. Despite the obvious importance of RpoS, negative selection for rpoS has been reported to take place in starved cultures, suggesting an adaptive occurrence allowing the overexpression of RpoD-dependent uptake and nutrient-scavenging systems. The trade-off of the RpoS-dependent functions for improved nutrient utilization abilities makes the bacterium more sensitive to environmental stressors, e.g., osmotic stress. In this work, we addressed the hypothesis that overinduction of genes in rpoS-deficient strains indicates their essentiality. Using DNA microarrays, real-time PCR, and transcriptional fusions, we show that genes of the wca operon, implicated in the production of the colanic acid exopolysaccharide, previously shown to be induced by osmotic stress, are also negatively controlled by RpoS. Both exopolysaccharides in the synthesis of which yjb and wca are involved are overproduced in an rpoS mutant during osmotic stress. We also show that both operons are essential in an rpoS-deficient strain but not in the wild type; promoters of both operons are constitutively active in yjb rpoS mutants; this strain produces extremely mucoid colonies, forms long filaments, and exhibits a reduced growth capability. In addition, the wca rpoS mutant's growth is inhibited by osmotic stress. These results indicate that although induced in the wild type, both operons are much more valuable for an rpoS-deficient strain, suggesting that the overproduction of both exopolysaccharides is an adaptive action.