Novel rkp gene clusters of Sinorhizobium meliloti involved in capsular polysaccharide production and invasion of the symbiotic nodule: the rkpK gene encodes a UDP-glucose dehydrogenase.
The production of exopolysaccharide (EPS) was shown to be required for the infection process by rhizobia that induce the formation of indeterminate nodules on the roots of leguminous host plants. In Sinorhizobium meliloti (also known as Rhizobium meliloti) Rm41, a capsular polysaccharide (KPS) analogous to the group II K antigens of Escherichia coli can replace EPS during symbiotic nodule development and serve as an attachment site for the strain-specific bacteriophage phi16-3. The rkpA to -J genes in the chromosomal rkp-1 region code for proteins that are involved in the synthesis, modification, and transfer of an as-yet-unknown lipophilic molecule which might function as a specific lipid carrier during KPS biosynthesis. Here we report that with a phage phi16-3-resistant population obtained after random Tn5 mutagenesis, we have identified novel mutants impaired in KPS production by genetic complementation and biochemical studies. The mutations represent two novel loci, designated the rkp-2 and rkp-3 regions, which are required for the synthesis of rhizobial KPS. The rkp-2 region harbors two open reading frames (ORFs) organized in monocistronic transcription units. Although both genes are required for normal lipopolysaccharide production, only the second one, designated rkpK, is involved in the synthesis of KPS. We have demonstrated that RkpK possesses UDP-glucose dehydrogenase activity, while the protein product of ORF1 might function as a UDP-glucuronic acid epimerase.
Institute of Genetics, Biological Research Center, Hungarian Academy of Sciences, H-6701 Szeged, Hungary.
A
A
Kereszt
E
E
Kiss
B L
BL
Reuhs
R W
RW
Carlson
A
A
Kondorosi
P
P
Putnoky
D016667:Bacterial Capsules / Q000096:biosynthesis*; D002238:Carbohydrate Epimerases / Q000235:genetics; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D008969:Molecular Sequence Data; D005810:Multigene Family; D016254:Mutagenesis, Insertional; D016366:Open Reading Frames; D009876:Operon / Q000235:genetics; D018517:Plant Roots / Q000382:microbiology*; D011135:Polysaccharides, Bacterial / Q000096:biosynthesis; D017386:Sequence Homology, Amino Acid; D016962:Sinorhizobium meliloti / Q000201:enzymology / Q000235:genetics*; D013559:Symbiosis / Q000235:genetics*; D014533:Uridine Diphosphate Glucose Dehydrogenase / Q000235:genetics*; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism
C095081:K polysaccharide, Rhizobium meliloti; D011135:Polysaccharides, Bacterial; D014535:Uridine Diphosphate Glucuronic Acid; D014533:Uridine Diphosphate Glucose Dehydrogenase; D002238:Carbohydrate Epimerases