Expression of extra-cellular levansucrase in Pseudomonas syringae is controlled by the in planta fitness-promoting metabolic repressor HexR

BACKGROUND: Pseudomonas syringae pv. glycinea PG4180 causes bacterial blight on soybean plants and enters the leaf tissue through stomata or open wounds, where it encounters a sucrose-rich milieu. Sucrose is utilized by invading bacteria via the secreted enzyme, levansucrase (Lsc), liberating glucose and forming the polyfructan levan. P. syringae PG4180 possesses two functional lsc alleles transcribed at virulence-promoting low temperatures. RESULTS: We hypothesized that transcription of lsc is controlled by the hexose metabolism repressor, HexR, since potential HexR binding sites were identified upstream of both lsc genes. A hexR mutant of PG4180 was significantly growth-impaired when incubated with sucrose or glucose as sole carbon source, but exhibited wild type growth when arabinose was provided. Analyses of lsc expression resulted in higher transcript and protein levels in the hexR mutant as compared to the wild type. The hexR mutant’s ability to multiply in planta was reduced. HexR did not seem to impact hrp gene expression as evidenced by the hexR mutant’s unaltered hypersensitive response in tobacco and its unmodified protein secretion pattern as compared to the wild type under hrp-inducing conditions. CONCLUSIONS: Our data suggested a co-regulation of genes involved in extra-cellular sugar acquisition with those involved in intra-cellular energy-providing metabolic pathways in P. syringae. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12866-015-0349-0) contains supplementary material, which is available to authorized users.