Molecular characterization of a gene region involved in lipopolysaccharide biosynthesis in Bradyrhizobium japonicum, soybean microsymbiont

초록

We have previously cloned and characterized a gene region which is involved in the synthesis of the B. japonicum lipopolysaccharide(LPS) molecule. In this study we have expanded sequence analysis of the region and found another open reading frame (orf) which appears to be divergently transcribed from the rfaF gene. Sequence alignment of the orf indicated similarity with rfaD genes of Salmonella typhimurium, Escherichia coli, and Neisseria gonorrhoeae. These genes encode a heptose-6-epimerase which catalyzes the interconversion of ADP-D-glycero-D-manno-heptose to ADP-L-glycero-D-manno-heptose. This divergent organization of the rfaF and rfaD genes is different from that of other gram-negative bacteria where two genes form an operon. A Salmonella rfaD- mutant was successfully transformed with a plasmid construct containing the rfaD gene from B. japonicum. Novobiocin sensitivity test of a wild-type, rfaD- and transformed rfaD- Salmonella strains showed the rfaD gene from B. japonicum could complement the mutation in Salmonella rfaD- strain.