α-Hydroxyketone Synthesis and Sensing by Legionella and Vibrio

Bacteria synthesize and sense low molecular weight signaling
molecules, termed autoinducers, to measure their population density
and community complexity. One class of autoinducers, the
α-hydroxyketones (AHKs), is produced and detected by the
water-borne opportunistic pathogens Legionella pneumophila and
Vibrio cholerae, which cause Legionnaires' disease and cholera,
respectively. The "Legionella quorum sensing" (lqs) or "cholera
quorum sensing" (cqs) genes encode enzymes that produce and sense
the AHK molecules "Legionella autoinducer-1" (LAI-1;
3-hydroxypentadecane-4-one) or cholera autoinducer-1 (CAI-1;
3-hydroxytridecane-4-one). AHK signaling regulates the virulence of
L. pneumophila and V. cholerae, pathogen-host cell interactions,
formation of biofilms or extracellular filaments, expression of a
genomic "fitness island" and competence. Here, we outline the
processes, wherein AHK signaling plays a role, and review recent
insights into the function of proteins encoded by the lqs and cqs
gene clusters. To this end, we will focus on the autoinducer
synthases catalysing the biosynthesis of AHKs, on the cognate
trans-membrane sensor kinases detecting the signals, and on
components of the down-stream phosphorelay cascade that promote the
transmission and integration of signaling events regulating gene
expression.