SYMBIOTIC HOST-SPECIFICITY OF RHIZOBIUM-MELILOTI IS DETERMINED BY A SULFATED AND ACYLATED GLUCOSAMINE OLIGOSACCHARIDE SIGNAL

RHIZOBIA are symbiotic bacteria that elicit the formation on leguminous plants of specialized organs, root nodules, in which they fix nitrogen1. In various Rhizobium species, such as R. leguminosarum and R. meliloti, common and host-specific nodulation (nod) genes have been identified which determine infection and nodulation of specific hosts'. Common nodABC genes2-5 as well as host-specific nodH and nodQ genes4,6-8 were shown recently, using bioassays, to be involved in the production of extracellular Nod signals. Using R. meliloti strains overproducing symbiotic Nod factors, we have purified the major alfalfa-specific signal, NodRm-1, by gel permeation, ion exchange and C18 reversephase high performance liquid chromatography. From mass spectrometry, nuclear magnetic resonance, 35S-labelling and chemical modification studies, NodRm-1 was shown to be a sulphated β-1,4-tetrasaccharide of D-glucosamine (Mr 1,102) in which three amino groups were acetylated and one was acylated with a C16, bisunsaturated fatty acid. This purified Nod signal specifically elicited root hair deformation on the homologous host when added in nanomolar concentration.