VfcA mediates amino acid chemotaxis in Vibrio fischeri 1 2

23 24 Flagellar motility and chemotaxis by Vibrio fischeri are important behaviors mediating 25 colonization of its mutualistic host, the Hawaiian bobtail squid. However, none of the 43 26 putative methyl-accepting chemotaxis proteins (MCPs) encoded in the V. fischeri genome has 27 been previously characterized. Using both an available transposon mutant collection and 28 directed mutagenesis, we isolated mutants for 19 of these genes, and screened them for altered 29 chemotaxis to six previously identified chemoattractants. Only one mutant was defective in 30 responding to any of the tested compounds; the disrupted gene was thus named vfcA (Vibrio 31 fischeri chemoreceptor A; locus tag VF_0777). In soft-agar plates, mutants disrupted in vfcA did 32 not exhibit the serine-sensing chemotactic ring, and the pattern of migration in the mutant was 33 unaffected by the addition of exogenous serine. Using a capillary chemotaxis assay, we showed 34 that, unlike wild-type V. fischeri, the vfcA mutant did not undergo chemotaxis towards serine, 35 and that expression of vfcA on a plasmid in the mutant was sufficient to restore the behavior. In 36 addition to serine, we demonstrated that alanine, cysteine, and threonine are strong attractants for 37 wild-type V. fischeri and that the attraction is also mediated by VfcA. This study thus provides 38 the first insights into how V. fischeri integrates information from one of its 43 MCPs to respond 39 to environmental stimuli. 40