Paraburkholderia phymatum σ54 controls utilization of dicarboxylates, motility and interbacterial competition

Paraburkholderia phymatum is a beta-proteobacterium, which lives in the soil and is able to enter nitrogen-fixing symbiosis with different legumes. The biological nitrogen fixation (BNF) process is of great ecological and agronomic importance. We previously showed that the expression of the key P. phymatum BNF enzyme – the nitrogenase –is regulated by the sigma factor σ54 (or RpoN) inside root nodules. This study focused on identifying the σ54 regulon of P. phymatum grown in nitrogen limited conditions using RNA-Sequencing. Among the genes significantly down-regulated in absence of σ54 we found those coding for a C4-dicarboxylate transport system (Bphy_0225-27), a flagellar biosynthesis cluster (Bphy_2926-64) and one of the two type 6 secretion system (T6SS-b) present in P. phymatum genome (Bphy_5978-97). Indeed, the σ54 mutant was unable to grow on C4 dicarboxylates (fumarate, malate and succinate) as the sole carbon source and was less motile compared to the wild-type strain. Both defects were complemented by adding rpoN in trans. Additionally, using reporter fusions we confirmed that T6SS-b expression is regulated by σ54. Finally, a σ54 mutant was less competitive than its parental strain against P. diazotrophica, suggesting a role of σ54 in controlling interbacterial competition. Unraveling the role of the master regulator RpoN in P. phymatum in free-living conditions.