Employing the biparental exogenous plasmid isolation method, conjugative plasmids conferring mercury resistance were isolated from the microbial community of the rhizosphere of field grown alfalfa plants. Five different plasmids were identified, designated pSB101-pSB105. One of the plasmids, pSB102, displayed broad host range (bhr) properties for plasmid replication and transfer unrelated to the known incompatibility (Inc) groups of bhr plasmids IncP-1, IncW, IncN and IncA/C. Nucleotide sequence analysis of plasmid pSB102 revealed a size of 55 578 bp. The transfer region of pSB102 was predicted on the basis of sequence similarity to those of other plasmids and included a putative mating pair formation apparatus most closely related to the type IV secretion system encoded on the chromosome of the mammalian pathogen Brucella sp. The region encoding replication and maintenance functions comprised genes exhibiting different degrees of similarity to RepA, KorA, IncC and KorB of bhr plasmids pSa (IncW), pM3 (IncP-9), R751 (IncP-1 beta) and RK2 (IncP-1 alpha), respectively. The mercury resistance determinants were located on a transposable element of the Tn5053 family designated Tn5718. No putative functions could be assigned to a quarter of the coding capacity of pSB102 on the basis of comparisons with database entries. The genetic organization of the pSB102 transfer region revealed striking similarities to plasmid pXF51 of the plant pathogen Xylella fastidiosa.