ISOLATION AND CHARACTERIZATION OF ROPA HOMOLOGOUS GENES FROM RHIZOBIUM-LEGUMINOSARUM BIOVARS VICIAE AND TRIFOLII

ropA encodes a 36-kDa elater membrane protein of Rhizobium leguminosar um bv. viciae strain 248 which constitutes the low-M(r) part of antige n group III (R. A. de Maagd, I. H. M. Mulders, H. C. J. Canter Cremers , and B. J. J. Lugtenberg, J. Bacteriol. 174:214-221, 1992). We observ ed that genes homologous to ropA are present in strain 248 as well as in other R. leguminosarum strains, and we describe the cloning and cha racterization of two of these genes. Sequencing of a 2.2-kb BglII frag ment from R. leguminosarum bv. viciae strain 248 that hybridizes with ropA revealed one large open reading frame of 1,074 bp encoding a matu re protein of 38.096 kDa. Homology between this gene and ropA is 91.8% on the DNA level. Homology on the amino acid level is only 69.9% as a result of a frameshift. On the basis of homology and immunochemical c haracteristics, we conclude that this gene encodes the high-M(r) part of the outer membrane protein antigen group III that is repressed duri ng symbiosis. We named this gene ropA2. The second gene that we cloned was the ropA homologous gene of R. leguminosarum bv. trifolii strain LPR5020. Except for amino acid 43, the N-terminal part of the correspo nding protein appeared to he identical to the first 31 amino acids of RopA of strain 248. The transcription start sites of both genes were d etermined, and the promoter regions were compared with that of ropA of strain 248. No clear consensus sequence could be deduced. The relatio nship of ropA and ropA2 of R. leguminosarum bv. viciae strain 248 with two similar genes from Brucella abortus is discussed.