The Sinorhizobium (Ensifer) fredii HH103 rkp-2 region is involved in the biosynthesis of lipopolysaccharide and exopolysaccharide but not in K-antigen polysaccharide production

et al.[Background]: Rhizobial surface polysaccharides are important molecular determinants required for successful symbiosis with legumes. In Sinorhizobium (Ensifer) meliloti Rm41, the rkp-2 region is involved in the biosynthesis of K-antigen polysaccharide (KPS) and lipopolysaccharide (LPS). This region is composed of two genes, lpsL and rkpK, which are respectively responsible for the production of galacturonic and glucuronic acid. [Results]: In this work, we show that in S. (Ensifer) fredii HH103 these genes do not form a transcriptional unit and that the transcriptional rate of rkpK is much higher than that of lpsL. Inactivation of each of these genes resulted in alterations in LPS, but did not affect KPS production, which is in agreement with the lack of uronic acids in S. fredii HH103 KPS. Mutation of rkpK also impaired HH103 exopolysaccharide (EPS) production, most probably due to the presence of glucuronic acid in HH103 EPS, as well as increased bacterial autoaggregation and osmosensitivity and decreased biofilm formation on plastic surfaces. Inactivation of rkpK affected negatively symbiosis with cowpea but not with soybean. Mutation of lpsL led to a complete symbiotic impairment with cowpea, whereas soybean plants inoculated with this mutant only formed pseudonodules. In both plants, the lpsL mutant showed defects in root infection. [Conclusion]: These results confirm the symbiotic importance of HH103 LPS in symbiosis with legumes.This work was supported by grants from the Spanish Ministry of Science and Innovation (BIO2011-30229 and BIO2016-78409-R) and the Andalusia Government (P07-CVI07500). SAJ and PNG are recipients of PhD grants from the VPPI of the University of Seville.Peer Reviewe