Mesorhizobium huakuii 7653R forms a symbiotic relationship with Astragalus sinicus to produce nitrogen-fixing root nodules under nitrogen-limiting conditions. By screening its transposon-inserted mutant library, a mutation in the lpsH gene was found to form pseudonodules on A. sinicus. Its effect was further confirmed by double-crossover mutant HK242. DNA sequence analysis revealed that the lpsH gene encodes a deduced protein of 397 amino acids with a predicted molecular mass of 43.6 kD. LpsH contains a putative signal peptide, 11 transmembrane helices, and an O-antigen polymerase domain, which locates on the periplasmic membrane and is necessary to lipopolysaccharide synthesis. Plant studies showed that the lpsH gene mutant formed ineffective nodules, and this symbiotic phenotype was linked to abnormal nodule development.