Large plasmid pCAR2 and class II transposon Tn4676 are functional mobile genetic elements to distribute the carbazole/dioxin-degradative car gene cluster in different bacteria

The carbazole-catabolic plasmid pCAR1 isolated from Pseudomonas resinovorans strain CA10 was sequenced in its entirety; and it was found that pCAR1 carries the class II transposon Tn4676 containing carbazole-degradative genes. In this study, a new plasmid designated pCAR2 was isolated from P. putida strain HS01 that was a transconjugant from mating between the carbazole-degrader Pseudomonas sp. strain K23 and P. putida strain DS1. Southern hybridization and nucleotide sequence analysis of pCAR1 and pCAR2 revealed that the whole backbone structure was very similar in each. Plasmid pCAR2 was self-transmissible, because it was transferred from strain HS01 to P. fluorescens strain IAM12022 at the frequency of 2×10^–7 per recipient cell. After the serial transfer of strain HS01 on rich medium, we detected the transposition of Tn4676 from pCAR2 to the HS01 chromosome. The chromosome-located copy of Tn4676 was flanked by a 6-bp target duplication, 5-AACATC-3. These results experimentally demonstrated the transferability of pCAR2 and the functionality of Tn4676 on pCAR2. It was clearly shown that plasmid pCAR2 and transposon Tn4676 are active mobile genetic elements that can mediate the horizontal transfer of genes for the catabolism of carbazole.