The biphenyl and salicylate metabolic pathways in Pseudomonas putida KF715 are chromosomally encoded. The bph gene cluster coding for the conversion of biphenyl to benzoic acid and the sal gene cluster coding for the salicylate meta-pathway were obtained from the KF715 genomic cosmid libraries. These two gene clusters were separated by 10-kb DNA and were highly prone to deletion when KF715 was grown in nutrient medium. Two types of deletions took place at the region including only the bph genes (ca. 40 kb) or at the region including both the bph and sal genes (ca. 70 kb). A 90-kb DNA region, including both the bph and sal genes (termed the bph-sal element), was transferred by conjugation from KF715 to P. putida AC30. Such transconjugants gained the ability to grow on biphenyl and salicylate as the sole sources of carbon. The bph and sal element was located on the chromosome of the recipient. The bph-sal element in strain AC30 was also highly prone to deletion; however, it could be mobilized to the chromosome of P. putida KT2440 and the two deletion mutants of KF715.A number of biphenyl-utilizing bacteria have been isolated to date. They include gram-negative species of Pseudomonas, Achromobacter, Sphingomonas, Comamonas, Burkholderia, Ralstonia, and Alcaligenes and gram-positive Rhodococcus spp. (1,3,11,37). Because these biphenyl-utilizing strains cometabolize polychlorinated biphenyls (PCB), the biochemistry of PCB degradation has been extensively studied (12). A gene cluster coding for biphenyl-PCB degradation (termed bph) was first cloned from Pseudomonas pseudoalcaligenes KF707 (14). Since then, a number of bph genes have been cloned and sequenced. Southern and sequence analyses of the bph genes revealed that some biphenyl-utilizing strains possess bph genes that are very similar, if not identical, to one another, but some share various degrees of homology (13).The bph genes are present on bacterial chromosomes (8, 14, 16, 35), plasmids (18, 39), and transposons (22, 28, 34). The presence of similar genes in different strains implies that even chromosomal bph genes have or used to have a mechanism for mobilization to other strains. The bph genes of Pseudomonas sp. strain CB406 were mobilized following the construction in vivo of a cointegrate plasmid inserted into the broad-hostrange plasmid RP4 (22). Springael and coworkers (23, 34) identified a transposon, Tn4371, carrying the bph genes encoding conversion of biphenyl to benzoic acid from Ralstonia eutrophus A5 (formerly Alcaligenes eutrophus A5) in which Tn4371 first transposed from the chromosome to indigenous IncP1 plasmid, and the plasmid carrying Tn4371 can be transferred to other strains by conjugation. A recent study shows that Tn4371 is a kind of conjugative transposon of 55 kb (24, 29). Interestingly, another smaller conjugative transposon Tnbph coding for biphenyl catabolism resides within Tn4371. This can be transferred to the recipient strain by conjugation independently. Divergence of the bph genes among various biphenyl-utilizing strains in...
