Cyclic di-GMP-mediated regulation of the Pga exopolysaccharide secretion machinery in "Escherichia coli"

The ubiquitous bacterial second messenger c-di-GMP plays a central role as a controller of the bacterial ‘lifestyle’ transition. High cellular c-di-GMP levels promote surface attachment and biofilm formation, while low levels favor single cell behavior, motility and virulence. In response to largely unknown internal and environmental cues, c-di-GMP is synthesized by diguanylate cyclases (DGCs) and turned over by specific phosphodiesterases (PDEs). Within a multicellular biofilm structure, bacteria are enclosed in a self-produced extracellular polymeric matrix that is mainly composed of different exopolysaccharides (EPS). Biofilms formed by the model organism Escherichia coli that are based on one specific EPS, poly-beta-1,6-N-acetylglucosamine (poly-GlcNAc), are in the main focus of this work. The cellular processes that underlie the induction of poly-GlcNAc-dependent biofilm formation in response to external stress are addressed at first, while the main part of this work focuses on the investigation of the molecular mechanisms behind c-di-GMP-stimulated poly-GlcNAc production.
In Boehm & Steiner et al. (2009), the molecular principles triggering poly-GlcNAc-dependent biofilm induction in response to subinhibitory concentrations of translation inhibitors were investigated. We present evidence that translational stress at the ribosome causes a SpoT-mediated reduction of ppGpp levels that results in the specific derepression of PgaA, an essential component of the poly-GlcNAc secretion machinery. In addition, we show that c-di-GMP that is mainly produced by the dedicated DGC YdeH strongly stimulates poly-GlcNAc-dependent biofilm formation and upregulates PgaD, another essential component of the secretion machinery. Evidence is provided that ppGpp and c-di-GMP play synergistic roles in controlling the poly-GlcNAc secretion machinery on a post-transcriptional level.
In the second and main part of this work, we show that the Pga machinery, which is involved in the production and secretion of poly-GlcNAc, is allosterically activated by c-di-GMP and we identify the inner membrane-bound PgaCD complex composed of the glycosyltransferase PgaC and the small protein PgaD as a novel type c-di-GMP receptor.