Metabolic role of LuxS, putative autoinducer-2 synthase, in Erwinia amylovora

The putative autoinducer-2 (AI-2) synthase gene LuXS was sequenced from Erwinia amylovora. Phylogenetic analysis indicated conservation of LuxS among genotypically diverse E. amylovora strains. Quorum sensing mediated by LuxS, has been implicated in coordinated gene expression, growth and virulence in other enterobacteria, but our evidence suggests that this is not the function in E. amylovora. Mutational analysis pointed to a role in colonization of apple blossoms, the primary infection court for fire blight but little or no role in virulence on apple shoots and pear fruit. Expression of key virulence genes, hrpL and dspA/E, was reduced in mutants of two E. amylovora strains, but stronger up-regulatory effects were observed for expression of metabolic genes involved in the activated methyl cycle (AMC) in those two mutants. Supplementation with key sulfur amino-acids in the AMC pathway complemented likely disruptions in the mutant. Despite a positive reaction with the AI-2 biosensor Vibrio harveyi BB170, quorum sensing was not confirmed in co-culture experiments with wild-type and mutant strains either in vitro or in apple blossoms. Known receptors essential for AI-2 quorum sensing, the LuxPQ sensor kinase or the Lsr ABC transporter, are absent in E. amylovora (and in many LuxS carrying bacteria) further indicating a primarily metabolic role for LuxS in this and other bacteria.