A genetic locus necessary for rhamnose uptake and catabolism in Rhizobium leguminosarum bv. trifolii

Rhizobium leguminosarum bv. trifolii is a gram-negative aerobic soil bacterium that can exist as a free-living heterotropic saprophyte or can form nitrogen-fixing nodules on various species of clovers (Trifolium spp.). The interaction of rhizobia with their hosts is a sequence of events that begins with an exchange of signals in the rhizosphere, followed by the invasion of the plant through infection threads and ultimately by release of the bacteria into plant cells, where they differentiate into nitrogen-fixing bacteroids. Through this symbiotic association, the plant provides the bacteria with energy for growth, and in return the Rhizobium provides the plant with fixed nitrogen. Although a great deal has been elucidated regarding the actual steps during nodulation (27, 56) and the metabolic pathways utilized while in the bacteroid state (39), comparatively little is known about what influences the growth of the bacteria prior to or during their interaction with plant roots. The rhizosphere has been described as the region directly under the influence of secreted compounds from the plant root (12). Competition for nodule occupancy exists between strains of Rhizobium within the rhizosphere and has been well documented in the literature (16, 54). A large number of biotic and abiotic factors influence competition between strains for nodule occupancy. Some of the best-characterized strain-dependent factors include production of antibiotics and bacteriocins (36, 42, 57) and the ability to catabolize specific compounds present in the root environment (11, 13, 18, 22, 23, 35, 43, 49, 55), as well as the synthesis or utilization of specific vitamins (38, 50). Rhamnose is a methyl-pentose sugar and is found as a constituent of pectin in the form of rhamnogalacturonan within the cell walls of dicotyledonous plants (28). It has also been found in the mucilage of a number of legume plants (26). R. leguminosarum mutants unable to utilize rhamnose as a sole carbon source were found to be significantly impaired in their nodulation competitiveness (35). The reason for the uncompetitive nature of these mutants is not understood. It was hypothesized that if the genetic region responsible for the catabolism of rhamnose were characterized, a better understanding of why these mutants were uncompetitive could be achieved. The objective of the present work, therefore, was to identify and characterize the genes in this region.