Characterization of Two Cyclic di-GMP Phosphodiesterases Hypothesized to Regulate Virulence in Streptomyces scabies

Common scab is a devastating disease that affects the marketability of potatoes. The disease results in raised and pitted lesions seen on the surface of the potato and is caused by the soil pathogen, Streptomyces scabies. The S. scabies genome has two cyclic-di-GMP phosphodiesterases, RmdA and RmdB, both regulators of morphology and development. These proteins break down the ubiquitous second messenger molecule cyclic di-GMP, a molecule in charge of cell cycle progression and virulence. Both proteins contain diguanylate cyclase domains and phosphodiesterase domains, GGDEF and EAL, respectively, while RmdA contains an additional PAS_4 domain. Despite similar domain configurations, the two appear to have little homology. Amino acid sequence alignments indicate there is only 35.47% identity between them. The two also differ structurally, unlike RmdA, RmdB has six transmembrane regions. BLAST searches reveal both RmdA and RmdB are highly conserved amongst streptomycetes and those species outside of the genus. Nothing is known regarding the influence of phosphodiesterases on the virulence of S. scabies. Arabidopsis thaliana seedlings were inoculated with the both the wild-type and ∆rmdB pathogenic strains of S. scabies. When compared to the wild-type, the mutant strain showed less severe infection. Seedlings inoculated with rmdB removed had longer roots and more cotyledon development than the wild-type. Preliminarily, the differences seen provide evidence of phosphodiesterase involvement in virulence. Understanding how these proteins affect the virulence of S. scabies can allow for improved treatment and prevention of the disease