IDENTIFICATION OF A MULTIDRUG AND TOXIC EXTRUSION TRANSPORTER INVOLVED IN THE SECRETION OF INDOLE-3-ACETIC ACID AND ITS CONJUGATES IN PSEUDOMONAS SAVASTANOI.

Pseudomonas savastanoi (Psv) is the causal agent of olive and oleander knot disease, characterised by hyperplastic galls on the aerial parts, and whose development is dependent on a functional type III secretion system (TTSS) and on the expression of hrp genes. Several phytohormones produced by Psv, i.e. the auxin 3-indoleacetic acid (IAA) and the cytokinin trans-zeatin, are also involved in knot formation. Psv synthesizes IAA from tryptophan, by the sequential activity of the enzymes IAAM and IAAH encoded by the homonymous genes. In Psv the iaaL gene is also present, coding for an enzyme for conjugation of IAA to lysine to give IAA-Lys. In the genome of the oleander strain Psn23, the iaaM/iaaH operon is located close to the iaaL gene, which upstream has an ORF encoding a putative multidrug and toxic compound extrusion (MATE) transporter (named matE). The IAA-driven inhibition of hrp genes, as well as data from phenotype microarray analysis on ΔiaaM, ΔiaaLand ΔmatE mutants, strongly suggested a functional link existing in Psv between IAA metabolism, TTSS and MATE. Here, through an integrated approach among bioinformatic analysis, structural biology and site-directed mutagenesis on Psn23 MATE, and HPLC-MS analysis of bacterial auxin production, we found the transporter encoded by matE contributing to this picture, by mediating IAA and IAA-Lys efflux. This is the first report of a MATE responsible for auxin transport in bacteria. Because of its role on IAA homeostasis in Psv, these findings could also contribute to the development of novel anti-infectives targeting Psv MATE.