Comparison of Four Comamonas Catabolic Plasmids Reveals the Evolution of pBHB To Catabolize Haloaromatics
ABSTRACTComamonasplasmids play important roles in shaping the phenotypes of their hosts and the adaptation of these hosts to changing environments, and understanding the evolutionary strategy of these plasmids is thus of great concern. In this study, the sequence of the 119-kb 3,5-dibromo-4-hydroxybenzonitrile-catabolizing plasmid pBHB fromComamonassp. strain 7D-2 was studied and compared with those of three otherComamonashaloaromatic catabolic plasmids. Incompatibility group determination based on a phylogenetic analysis of 24 backbone gene proteins, as well as TrfA, revealed that these four plasmids all belong to the IncP-1β subgroup. Comparison of the four plasmids revealed a conserved backbone region and diverse genetic-load regions. The four plasmids share a core genome consisting of 40 genes (>50% similarities) and contain 12 to 50 unique genes each, most of which are xenobiotic-catabolic genes. Two functional reductive dehalogenase gene clusters are specifically located on pBHB, showing distinctive evolution of pBHB for haloaromatics. The higher catabolic ability of thebhbA2B2cluster than thebhbABcluster may be due to the transcription levels and the character of the dehalogenase gene itself rather than that of its extracytoplasmic binding receptor gene. The plasmid pBHB is riddled with transposons and insertion sequence (IS) elements, and ISs play important roles in the evolution of pBHB. The analysis of the origin of thebhbgenes on pBHB suggested that these accessory genes evolved independently. Our work provides insights into the evolutionary strategies ofComamonasplasmids, especially into the adaptation mechanism employed by pBHB for haloaromatics.