Phenylacetyl-Homoserine-Lactone Quorum Sensing In Prosthecomicrobium Hirschii

Many species of Proteobacteria synthesize acyl-homoserine lactone(acyl-HSL)compounds for use as signals in cell density-dependent cell-cell communication known as quorum sensing(QS).Most acyl-HSL signal synthases,LuxI-type enzymes,produce fatty acyl-HSL signals and with their fatty acid moieties derived from corresponding acyl-acyl carrier protein(ACP)substrates from fatty acid biosynthesis.More recently,a subclass of LuxI homologs now have been identified that utilize coenzyme A(CoA)-linked substrates to synthesize acyl-HSL signals with aromatic acid or branched amino acid side chains.These CoA-type LuxI homologs now have their own separate enzyme commission category,facilitating bioinformatics analysis of the prevalence of such enzymes in proteobacterial genomes.This study found that CoA-type LuxI homologs are mainly restricted to the ?-proteobacteria,which are more common than previously appreciated,especially among the plant-associated bacterial species.Among them,the genome of Prosthecomicrobium hirschii,which is a prostecate ?-proteobacterium.Contain the genes encoding a putative CoA-type luxI synthase(HirI)and a paired receptor(HirR).Further studies showed that P.hirschii produced a novel aryl QS signal phenylacetyl-homoserine lactone(PA-HSL)at a moderately high level(~300 nM)when the plant hormone phenylacetate is added to the growth medium.HirI catalyzed the synthesis of PA-HSL presumably via phenylacetyl-CoA.HirR responded to nM levels of PA-HSL to activate gene transcription.Using an acyl-HSL-degrading enzyme to create “QS-off” conditions,it was found that PA-HSL influenced cell clumping,biofilm formation and production of extracellular polymeric substances and pigments.Our results suggest that there is a large amount of untapped diversity of acyl-HSL-type signals among the ?-proteobacteria and sequence analyses provides a means of identifying them.