Identification of a Conserved, Reiterated DNA Region That Influences the Efficiency of Nodulation in Strain RS1051 of Rhizobium leguminosarum bv. trifolii. Francisco Rodríguez-Quiñones. Department of Microbiology, Faculty of Pharmacy, University of Seville, E-41012 Seville, Spain. Miguel Fernández-Burriel(1), Zsofia Banfalvi(2), Manual Megías(1), and Adam Kondorosi(2). (1)Department of Microbiology, Faculty of Pharmacy, University of Seville, E-41012 Seville, Spain, and (2) Institute of Genetics, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary. . MPMI 2:75-83. Accepted 29 December 1988. Copyright 1989 The American Phytopathological Society.

The symbiotic plasmid of strain RS1051 of Rhizobium leguminosarum bv. trifolii has been identified by: an indirect approach through isolation of deleted and cured derivatives, mobilization of the plasmid into Agrobacterium, and hybridization with nod and nif gene probes. Two cosmids carrying the RS1051 nod region were selected from a genomic clone bank. Subcloning and deletion analysis indicate that an 11.45-kb DNA region on the symbiotic plasmid carries all the essential genes for red and white clover nodulation in R. l. bv. trifolii and for red clover nodulation in the heterologous strains Agrobacterium and R. l. bv. viceae, whereas an additional 2.55-kb region has been proven to be necessary for white clover nodulation by those hosts. In addition, a 1.7-kb region located adjacent to the nodFE genes has been found to influence the efficiency of nodulation of both red and white clover. This region is structurally conserved among the rhizobia examined and structurally as well as functionally conserved in R. l. bv. viceae. In R. l. bv. trifolii RS1051 the 1.7-kb nodlocus is reiterated on the pSym, and our results indicate that at least two of the copies are functional and necessary for successful nodulation. Furthermore, evidence is presented that strongly indicates that the RS1051 nodD gene is functionally reiterated and works in a host-specific manner.