A widespread methylotroph acyl-homoserine lactone synthase produces an atypical quorum sensing signal

Pink pigmented facultative methylotrophs of the genera Methylorubrum and Methylobacterium are omnipresent bacteria often found associated with plants. Despite their widespread occurrence, the molecular details of how these organisms interact with each other and their environment remain understudied. We analyzed genes encoding N-acylhomoserine lactone quorum sensing signal synthases in published genomes of these bacteria and determined that the product of the largest group of signal synthases had not been characterized. We subsequently identified this N-acylhomoserine lactone product using inverse stable isotopic labeling (InverSIL), which revealed an atypical signal. We then demonstrate in one representative strain that this signal activates its cognate LuxR-family transcription factor and is produced in a positive feedback loop. These results reveal a previously undescribed yet widespread signal used by pink pigmented facultative methylotrophs, which helps us understand the chemical ecology of these important bacteria.