Proteins from Cells of Rhizobium fredii Bind to DNA Sequences Preceding noIX, a Flavonoid-lnducible nod Gene that Is Not Associated with a nod Box. Claudia M Bellato. Department of Plant Pathology, University of Missouri, Columbia 65211 U.S.A. Pedro A. Balatti, Steven G. Pueppke, and Hari B. Krishnan. Department of Plant Pathology, University of Missouri, Columbia 65211 U.S.A. MPMI 9:457-463. Accepted 29 March 1996. Copyright 1996 The American Phytopathological Society.

Rhizobium fredii strains USDA257 and USDA191 both contain a set of nodulation genes termed nolXWBTUV. In the USDA257 background, noIX prevents infection of soybean cultivars such as McCall, and in both backgrounds, it blocks nodulation of Erythrina spp. We report here that expression of noIX is differentially responsive to a panel of flavonoids, and that the most potent inducers are also the most active inducers of nodC, a conventional, nod box-associated gene. Cell-free protein extracts from uninduced and flavonoid-induced cells of strains USDA191 and USDA257 retard the cleclrophorctic mobility of DNA sequences that lie upstream of noIX. Binding is dependent both on nodDl and nodD2, and it is abolished by the presence of a double-stranded, 23-bp oligonucleotide that lies within a 114-bp Taql/Sacll restriction fragment. This oli-gomer has significant sequence homology to A3, a putative negative regulatory element from R. leguminosarum bv. viciae. Deletion of the A3-homologous sequences elevates the basal and flavonoid-inducible expression of noIX by about 50%