Sinorhizobium fredii USDA257, a cultivar-specific soybean symbiont, carries two copies of y4yA and y4yB, two open reading frames that are located in a region that encodes the type III protein secretion system

Sinorhizobium fredii USDA257 forms nitrogen-fixing nodules on primitive soy bean (Glycine max) cultivar Peking but fails to nodulate the improved culti var McCall. Cultivar specificity is governed by a plasmid-borne locus, nolX BTUV. By DNA sequence analysis, we have identified two open reading frames, y4yA and y4yB, immediately downstream of nolX, Northern (RNA) blot analysi s indicated that the expression of both y4yA and y4yB is inducible by isofl avonoids, and an intact copy of nolX is required. Two copies each of y4yA a nd y4yB are present in S,fredii USDA257, one on the sym plasmid (y4yAsp and y4yBsp), and the other on the chromosome (y4yAc and y4yBc). The cultivar-n onspecific strain USDA191 lacks y4yAc and y4yBc, Introduction of y4yAc plus y4yBc from USDA257 into USDA191 did not influence the ability of the latte r strain to nodulate McCall soybean plants, Unlike nolX, the inactivation o f y4yAsp and y4yBsp of USDA257 did not extend the host range of this strain , A double mutant, in which both the plasmid and chromosomal copies of y4yA and y4yB were mutated, had no observable effect on symbiotic ability of US DA257, The y4yAsp and y4yBsp mutants did not influence flavonoid-dependent extracellular protein production. Rhizobium sp, strain NGR234 and S, saheli USDA4893 both contain sequences similar to S, fredii USDA257 y4yAsp and y4 yBsp; however, Bradyrhizobium spp,, the traditional soybean symbionts, lack these genes.