The second messenger cyclic di-GMP (c-di-GMP) is a nearly ubiquitous intracellular signal molecule known to regulate various cellular processes including biofilm formation, motility, and virulence. This intracellular concentration of this molecule is inversely governed by GGDEF-containing proteins (GCPs) and EAL-containing proteins (ECPs), which synthesize and degrade c-di-GMP, respectively. The role of this regulatory molecule in the plant pathogen and causal agent of fire blight disease, Erwinia amylovora was explored. In this study, it is demonstrated that E. amylovora contains three functional diguanylate cyclases (DGCs) that synthesize c-di-GMP: gcp1, gcp3, and gcp5; and two functional c-di-GMP specific phosophodiesterases (PDEs) that degrade c-di-GMP: ecp1 and ecp3. C-di-GMP was not detected in the wild type strain. C-di-GMP was shown to positively regulate biofilm formation and secretion of the main expolysaccharide amylovoran and to inversely regulate swimming motility and virulence. A deletion of ecp3 resulted in a hyper-biofilm forming phenotype. Interestingly, over-expression of dgc1, which contains both an EAL and a GGDEF domain, resulted in no observable phenotypic differences from the wild type strain, even though over-expression of dgc1 increased the levels of c-di-GMP.
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