EVIDENCE FOR 2 CHEMOSENSORY PATHWAYS IN RHODOBACTER-SPHAEROIDES

In contrast to enteric bacteria, chemotaxis in Rhodobacter sphaeroides requires transport and partial metabolism of chemoattractants, Althou gh a chemotaxis operon has been identified containing homologues of th e enteric cheA, cheW, cheR genes and two homologues of the cheY gene, deletion of the entire chemotaxis operon had only minor effects on che motactic behaviour under the conditions tested. Responses to sugars we re enhanced in tethered cells but in all other chemotaxis assays behav iour of the operon deletion mutant was wild type, The mutant also show ed wild-type responses to weak organic acids such as acetate and propi onate, the dominant chemoattractants for this organism, under all cond itions. This is in direct contrast to the enterics in which CheA, CheW and CheY are absolutely essential for taxis to PTS sugars, oxygen and MOP-dependent chemoeffectors, The operon deletion mutant was subjecte d to Tn5 transposon mutagenesis and new mutants selected using a chemo taxis and phototaxis screen, One mutant, JPA203, was non-chemotactic o n swarm plates and showed inverted responses when tethered or subjecte d to changes in light intensity, Characterization of the Tn5 insertion in JPA203 identified a second chemotaxis operon in R. sphaeroides tha t contains homologues of cheY, cheA and cheR, the first homologue of c heB and two homologues of cheW. The new genes were labelled orf10, che Y(III), cheA(II), cheW(II), cheW(III), cheR(II), cheB and tlpC, When i ntroduced into a wild-type background, deletion of cheA(II) produced a chemotaxis minus phenotype in R. sphaeroides, suggesting that cheA(II ) forms part of a dominant chemotactic pathway, whereas the earlier id entified operon plays only a minor role under laboratory conditions, T he data presented here support the existence of two chemosensory pathw ays in R. sphaeroides, a feature that so far is unique in bacterial ch emotaxis.