lpsB a Symbiotically Relevant Gene for R. meliloti is also Present and Functional in other Rhizobial Species

Abstract

Rhizobial LPSs are among the bacterial surface compounds relevant for root infection in different symbiotic systems. In our laboratory we have characterized a Rhizobium meliloti lpsB mutant which was altered in competitiveness for nodulation of Medicago sativa and presented a Fix ⁻ phenotype in M. trimcatula. A significant DNA homology was detected between the Rme lpsB sequence and a DNA region map** downstream the dctA gene of R. leguminosarum bv viciae. From this evidence we investigated: a) if lpsB homologous DNA sequences are also present in other rhizobia b) if these sequences are involved in LPS synthesis, and c) if they are relevant for symbiosis. By a nested PCR assay using primers contained in the lpsB coding sequence, a conserved fragment of 267 bp could be amplified from Rmeliloti 2011, R. meliloti L5–30, R. meliloti Rm41, R. leguminosarum bv. viciae VF39, R. leguminosarum bv. trifolii ANU843, R. fredii USDA191, R. fredii USDA257, R.spp BR816, R. tropici CFN299, R. tropici CIAT899, R. etli CE3, R. spp. LPU83, A. tumefaciens LBA958, R. spp. GRH2 (Acacia). We further investigated whether these DNA sequences are in fact LPS associated genes. Using site-directed vector integration an lpsB mutant of R. etli CE3 was constructed. The CE3 lpsB mutant presented an altered DOC-PAGE LPS profile suggesting that lpsB is involved in LPS biosynthesis in R. etli. However, the CE3 lpsB mutant displayed a Nod⁺ Fix⁺ phenotype in common beans. Evidence here presented supports the possibility that functional lpsB homologous genes can also be present in the other PCR positive rhizobial species.