ABSTRACT The nucleotide sequence of the biphenyl catabolic transposon Tn 4371 has been completed and analyzed. It confirmed that the element has a mosaic structure made of several building blocks. In addition to previously identified genes coding for a tyrosine recombinase related to phage integrases and for biphenyl degradation enzymes very similar to those of Achromobacter georgiopolitanum KKS102, Tn 4371 carries many plasmid-related genes involved in replication, partition, and other, as-yet-unknown, plasmid functions. One gene cluster contains most of the genes required to express a type IV secretion-mating pair formation apparatus coupled with a TraG ATPase, all of which are related to those found on IncP and Ti plasmids. Orthologues of all Tn 4371 plasmid-related genes and of the tyrosine recombinase gene were found, with a very similar organization, in the chromosome of Ralstonia solanacearum and on the yet-to-be-determined genomic sequences of Erwinia chrysanthemi and Azotobacter vinelandii . In each of these chromosomal segments, conserved segments were separated by different groups of genes, which also differed from the Tn 4371 bph genes. The conserved blocks of genes were also identified, in at least two copies, in the chromosome of Ralstonia metallidurans CH34. Tn 4371 thus appears to represent a new family of potentially mobile genomic islands with a broad host range since they reside in a wide range of soil proteobacteria, including plant pathogens.