Evidence for two chemosensory pathways in Rhodobacter sphaeroides

Abstract

SummaryIn contrast to enteric bacteria, chemotaxis in Rhodobacter sphaeroides requires transport and partial metabolism of chemoattractants. Although a chemotaxis operon has been identified containing homologues of the enteric cheA, cheW, cheR genes and two homologues of the cheY gene, deletion of the entire chemotaxis operon had only minor effects on chemotactic behaviour under the conditions tested. Responses to sugars were enhanced in tethered cells but in all other chemotaxis assays behaviour of the operon deletion mutant was wild type. The mutant also showed wild-type responses to weak organic acids such as acetate and propionate, the dominant chemoattractants for this organism, under all conditions. This is in direct contrast to the enterics in which CheA, CheW and CheY are absolutely essential for taxis to PTS sugars, oxygen and MCP-dependent chemoeffectors. The operon deletion mutant was subjected to Tn5 transposon mutagenesis and new mutants selected using a chemotaxis and phototaxis screen. One mutant, JPA203, was non-chemotactic on swarm plates and showed inverted responses when tethered or subjected to changes in light intensity. Characterization of the Tn5 insertion in JPA203 identified a second chemotaxis operon in R. sphaeroides that contains homologues of cheY, cheA and cheR, the first homologue of cheB and two homologues of cheW. The new genes were labelled orf10, cheY III , cheA II , cheW II , cheW III , cheR II , cheB and tlpC. When introduced 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 identified operon plays only a minor role under laboratory conditions. The data presented here support the existence of two chemosensory pathways in R. sphaeroides, a feature that so far is unique in bacterial chemotaxis.

Chemotaxis in Rhodobacter sphaeroides Requires an Atypical Histidine Protein Kinase

“…The Importance of CheA Phosphorylation for ChemotaxisAlthough previous deletion studies have demonstrated that CheA 2 , CheA 3 , and CheA 4 are all essential for chemotaxis in R. sphaeroides (15,19), the precise reason for their requirement has not been determined. The CheAs may be required for either their role in phosphosignaling and/or their role in localizing other components of the chemotaxis signaling pathway to either the cell poles or the cytoplasmic chemotaxis cluster.…”

“…These PCR products were cloned into the allelic exchange suicide vector pK18mobsacB to generate the mutation plasmids (Table I). The mutation plasmids were checked by sequencing and then used to introduce the point mutations into the genome of R. sphaeroides by allelic exchange as described previously (Table II) (19,31).…”

“…The chemotaxis system of R. sphaeroides is encoded in three major chemosensory loci (15,18,19). Although R. sphaeroides lacks a CheZ homologue, each locus encodes homologues of the remaining proteins of the E. coli chemosensory pathway.…”

“…Although R. sphaeroides lacks a CheZ homologue, each locus encodes homologues of the remaining proteins of the E. coli chemosensory pathway. The proteins of immediate relevance to this study are: CheA 1 , CheY 1 , CheY 2 , and CheY 5 encoded by cheOp 1 (18,20); CheA 2 , CheB 1 , and CheY 3 encoded by cheOp 2 (19); CheA 3 , CheA 4 , CheB 2 , and CheY 6 encoded by cheOp 3 (15); CheY 4 encoded in a separate genetic locus with the chemoreceptor McpG (21). Genetic studies have shown that, although CheA 2 , CheA 3 , CheA 4 , CheB 1 , CheB 2 , and CheY 6 are essential for chemotaxis (15,22,23), all of the proteins encoded by cheOp 1 have no detectable role in chemotaxis (18).…”

Sinorhizobial chemotaxis: a departure from the enterobacterial paradigm a aTo Professor Wolfram Heumann on his 87th birthday.

“…Is there a biological concept behind these duplicate pathways? In the photosynthetic Rhodobacter sphaeroides, one of two che operons is dominant directing all known photo-and chemotactic responses, whereas mutants in the second che operon have virtually no effect on bacterial behaviour under laboratory conditions (Hamblin et al, 1997). Similarly, in S. meliloti, the chromosomal che operon dominantly controls the response to all tested attractants (Greck et al, 1995 ;Sourjik & Schmitt, 1996).…”