Abstract

The regulatory gene merR, and the adjacent operator/promoter region was amplified from the mercury resistance (Hg(R)) determinants from 10 Gram-negative bacterial isolates from mercury polluted and pristine environments using the polymerase chain reaction. These mer regions showed polymorphism in size of PCR amplification products with those from isolates SE3, SE11, SE12, SE31, SO1 and T217 being of 557 base pairs in size, whilst those from isolates SE20, T238, SB3, SB4 and the positive control (Tn501) were 536 base pairs in size. From the sequence analysis of these mer regions and comparison with previously sequenced Hg(R) determinants an evolutionary tree was constructed which showed there to be a significant difference between Gram-negative merR genes and those found in Gram-positive organisms. With the exception of the Thiobacillus Hg(R) determinants, merR genes from Gram negative bacteria were strongly conserved and could be grouped closely around the previously sequenced determinants of Tn501, Tn21, Tn5053 (pMER327/419) and pKLH2. Only the merR genes of pDU1358 and T238 showed significant variation from these subgroups. The regions of greatest variation were the carboxyl terminal coding region of the merR gene and the operator/promoter region. It is suggested that, due to the global nature of inducible mercury resistance and its strong sequence conservation across large geographical distances, bacterial resistance to mercury is an ancient genetic character.