ropA encodes a 36-kDa outer membrane protein of Rhizobium leguminosarum bv. viciae strain 248 which constitutes the low-M(r) part of antigen group III (R.A. de Maagd, I.H.M. Mulders, H.C.J. Canter Cremers, B.J.J. Lugtenberg, J. Bacteriol. 174:214-221, 1992). We observed that genes homologous to ropA are present in strain 248 as well as in other R. leguminosarum strains, and we describe the cloning and characterization of two of these genes. Sequencing of a 2.2-kb Bg/II fragment from R. leguminosarum bv. viciae strain 248 that hybridizes with ropA revealed one large open reading frame of 1,074 bp encoding a mature 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 characteristics, we conclude that this gene encodes the high-M(r) part of the outer membrane protein antigen group III that is repressed during 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 corresponding protein appeared to be identical to the first 51 amino acids of RopA of strain 248. The transcription start sites of both genes were determined, and the promoter regions were compared with that of ropA of strain 248. No clear consensus sequence could be deduced. The relationship of ropA and ropA2 of R. leguminosarum bv. viciae strain 248 with two similar genes from Brucella abortus is discussed
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