POLYMERASE CHAIN REACTION-RESTRICTION FRAGMENT LENGTH POLYMORPHISM ANALYSIS SHOWS DIVERGENCE AMONG MER DETERMINANTS FROM GRAM-NEGATIVE SOIL BACTERIA INDISTINGUISHABLE BY DNA-DNA HYBRIDIZATION

Mercury resistant (Hg(r)) bacteria were isolated from four terrestrial sites: three containing high levels of mercury (sites T2, SE, and SO) and one uncontaminated site (SB). The frequencies of Hg(r) bacteria in the total cultivable populations were 0.05% (SB), 0.69% (SO), 4.8% (SE), and 25% (T2). Between 35 and 100% of the isolates from the four sites contained DNA sequences homologous to a DNA probe from the mercury resistance (mer) operon of the Tn501 Hg(r) determinant. The mer sequences of 10 Tn501-homologous Hg(r) determinants from each site were amplified by the polymerase chain reaction, with primers designed to consensus sequences of the mer determinants of Tn501, Tn21, and pMH100, and were classified on the basis of the size of the amplified product and the restriction fragment length polymorphism pattern. Two main groups of amplification product were identified. The first, represented by the T2 and SB isolates and one SE isolate, gave an amplification product indistinguishable in size from that amplified from Tn501 (approximately 1,010 bp). The second group, represented by the SO isolates and the majority of the SE isolates, produced larger amplification products of 1,040 or 1,060 bp. Restriction fragment length polymorphism analysis revealed that each amplification product size group could be further subdivided into five subgroups.