nolMNO genes of Bradyrhizobium japonicum are co-transcribed with nodYABCSUIJ, and nolO is involved in the synthesis of the lipo-oligosaccharide nodulation signals

Abstract

A host-inducible lacZ fusion was mapped down-stream of the nodYABCSUIJ operon in Bradyrhizobium japonicum strain USDA110. Sequencing of this region identified three novel genes, nolMNO. RNA dot blot analysis showed that nolO transcription is nodD1-dependent and that a polar mutation in nodS, located 5 kilobases upstream of nolO, blocks the transcription of nolO. Coupled with the host-inducible nature of nolO expression, these results indicate that nolMNO are part of a 9-kilobase operon, nodYABCSUIJnolMNO. The lipo-oligosaccharide nodulation signals produced by strains SL67 (nolO-) and SL65(nolNO-) were purified, and their chemical structures were determined. In addition to the wild-type signal molecules, both mutants produced modified compounds that are not produced by the parent strain USDA110. The most prevalent difference observed was the absence of the 2-O-methylfucosyl residue from the mutant structures. In addition, metabolites were found in which the N-acetylglucosamine residue at the reducing end was glycosidically linked to glycerol. These alterations in the profiles of nodulation signals produced by strains SL67 and SL65 were accompanied by reduced nodulation efficiency on all hosts tested.