AmrZ is a major determinant of c-di-GMP levels in Pseudomonas fluorescens F113

The transcriptional regulator AmrZ is a global regulatory protein conserved within the pseudomonads. AmrZ can act both as a positive and a negative regulator of gene expression, controlling many genes implicated in environmental adaption. Regulated traits include motility, iron homeostasis, exopolysaccharides production and the ability to form biofilms. In Pseudomonas fluorescens F113, an amrZ mutant presents a pleiotropic phenotype, showing increased swimming motility, decreased biofilm formation and very limited ability for competitive colonization of rhizosphere, its natural habitat. It also shows different colony morphology and binding of the dye Congo Red. The amrZ mutant presents severely reduced levels of the messenger molecule cyclic-di-GMP (c-di-GMP), which is consistent with the motility and biofilm formation phenotypes. Most of the genes encoding proteins with diguanylate cyclase (DGCs) or phosphodiesterase (PDEs) domains, implicated in c-di-GMP turnover in this bacterium, appear to be regulated by AmrZ. Phenotypic analysis of eight mutants in genes shown to be directly regulated by AmrZ and encoding c-di-GMP related enzymes, showed that seven of them were altered in motility and/or biofilm formation. The results presented here show that in P. fluorescens, AmrZ determines c-di-GMP levels through the regulation of a complex network of genes encoding DGCs and PDEs

Dimerisation induced formation of the active site and the identification of three metal sites in EAL-phosphodiesterases

The bacterial second messenger cyclic di-3′,5′-guanosine monophosphate (c-di-GMP) is a key regulator of bacterial motility and virulence. As high levels of c-di-GMP are associated with the biofilm lifestyle, c-di-GMP hydrolysing phosphodiesterases (PDEs) have been identified as key targets to aid development of novel strategies to treat chronic infection by exploiting biofilm dispersal. We have studied the EAL signature motif-containing phosphodiesterase domains from the Pseudomonas aeruginosa proteins PA3825 (PA3825EAL) and PA1727 (MucREAL). Different dimerisation interfaces allow us to identify interface independent principles of enzyme regulation. Unlike previously characterised two-metal binding EAL-phosphodiesterases, PA3825EAL in complex with pGpG provides a model for a third metal site. The third metal is positioned to stabilise the negative charge of the 5′-phosphate, and thus three metals could be required for catalysis in analogy to other nucleases. This newly uncovered variation in metal coordination may provide a further level of bacterial PDE regulation

Structure and regulation of EAL domain proteins

The formation and dispersal of bacterial biofilms is strongly correlated with cellular levels of bis-(3′–5′) cyclic dimeric guanosine monophosphate, cyclic di-GMP, a secondary messenger that has been shown to be involved in regulation of a broad range of cellular processes in bacteria. Diguanylate cyclases (DGCs) are required for synthesis of cyclic di-GMP, with phosphodiesterases (PDEs) responsible for its breakdown. This review focuses on PDEs characterised by the presence of the conserved “EAL” sequence motif. Typically found in multi-domain proteins, EAL domains can couple to sensory or regulatory domains that allow their activity to be regulated by environmental stimuli or cellular cues. Additionally, catalytically inactive EAL PDEs are suggested to have a sensory or otherwise regulatory function. Recent structure determination provides a wealth of information on PDE function and regulation and has provided novel insight into the enzymatic reaction mechanism. Several regulatory layers may control activity, including dimerisation, active site formation, and metal coordination. In this review, we provide a concise summary of these exciting findings and highlight open research questions that will allow us in future to decipher many of the cellular signals responsible for regulation of PDE activity and cellular processes influenced by these pivotal enzymes