Silent Circuit Affects Metabolism in Escherichia coli K12

Abstract

The bgl operon encodes proteins for the utilization of β-glucoside sugars s such as salicin but is not expressed in wild-type cells. Either the overexpression of LeuO or BglJ results in activation of bgl expression. However, both leuO and bglJ are silenced by the H-NS protein as is the bgl operon. The retention of these genes provides a selective advantage against soil protozoa. In pathogenic bacteria, the bgl genes can be expressed but the activating signals are not known. To unravel this silent circuit, we have examined targets of BglJ. BglJ forms a heteromeric complex with RcsB, a regulator of the cell envelope and part of a stress response. These targets could titrate away the BglJ-RcsB complex and prevent the activation of the bgl operon, especially when in multiple copies. Using a genetics screen of an E. coli plasmid library, we found new silencers of the BglJ activated bgl operon, which include a truncated nanA gene. NanA is the first enzyme in sialic acid catabolism. We have narrowed the silencing region from 118 bp to 31 bp in nanA by cloning fragments and examining the Bgl phenotype. No obvious BglJ-RcsB binding sites are present. Whether this region of nanA silences through a mechanism involving DNA or RNA or both is currently under investigation. In addition, microarrays have implicated BglJ in the regulation of iron acquisition. We have verified one target, cirA, by qRT-PCR. How BglJ controls these genes and the ability to scavenge iron has implications for pathogenesis.