PLASMIDS RESPONSIBLE FOR HORIZONTAL TRANSFER OF NAPHTHALENE CATABOLISM GENES BETWEEN BACTERIA AT A COAL-TAR CONTAMINATED SITE ARE HOMOLOGOUS TO PDTG1 FROM PSEUDOMONAS-PUTIDA NCIB-9816-4

The presence of a highly conserved nahAc allele among phylogenetically diverse bacteria carrying naphthalene-catabolic plasmids provided evi dence for in situ horizontal gene transfer at a coal tar-contaminated site (J, B. Herrick, K, G. Stuart-Keil, W. C. Ghiorse, and E, L. Madse n, Appl, Environ. Micro-biol. 63:2330-2337, 1997), The objective of th e present study was to identify and characterize the different-sized n aphthalene-catabolic plasmids in order to determine the probable mecha nism of horizontal transfer of the nahAc gene in situ, Filter matings between naphthalene-degrading bacterial isolates and their cured proge ny revealed that the naphthalene catabolic plasmids were self-transmis sible. Limited interstrain transfer was also found. Analysis of the re striction fragment length polymorphism (RFLP) patterns indicated that catabolic plasmids from 12 site-derived isolates were closely related to each other and to the naphthalene-catabolic plasmid (pDTG1) of Pseu domonas putida NCIB 9816-4, which was isolated decades ago in Banger, Wales. The similarity among all site-derived naphthalene-catabolic pla smids and pDTG1 was confirmed by using the entire pDTG1 plasmid as a p robe in Southern hybridizations. Two distinct but similar naphthalene catabolic plasmids were retrieved directly from the microbial communit y indigenous to the contaminated site in a filter mating by using a cu red, rifampin-resistant site-derived isolate as the recipient, RFLP pa tterns and Southern hybridization showed that both of these newly retr ieved plasmids, like the isolate-derived plasmids, were closely relate d to pDTG1, These data indicate that a pDTG1-like plasmid is the mobil e genetic element responsible for transferring naphthalene-catabolic g enes among bacteria in situ. The pervasiveness and persistence of this naphthalene-catabolic plasmid suggest that it may have played a role in the adaptation of this microbial community to the coal tar contamin ation at our study site.