DISTRIBUTION, DIVERSITY AND EVOLUTION OF THE BACTERIAL MERCURY RESISTANCE (MER) OPERON

Mercury and its compounds are distributed widely across the earth. Man y of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cy cling of mercury in the natural environment. Five mechanisms of resist ance to mercury compounds have been identified, of which resistance to inorganic mercury (Hg-R) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistances to heavy metal s in general. Resistance to inorganic mercury is encoded by the genes of the met operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical dist ribution, and furthermore, are found across a wide range of both Gram- negative and Grampositive bacteria from both natural and clinical envi ronments. The presence of mer genes in bacteria from sediment cores su ggest that mer is an ancient system. Analysis of DNA sequences from me r operons and genes has revealed genetic variation both in operon stru cture and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has iden tified a number of vestigial non-functional operons. It is hypothesise d that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response t o increased levels of mercury in natural environments, perhaps resulti ng from volcanic activity. Models for the evolution of both a basic me r operon and for the Tn21-related family of mer operons and transposon s are suggested. The study of evolution in bacteria has recently becom e dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which in corporate the effects of horizontal gene transfer. In addition, the me r operon provides a model system in the study of environmental microbi ology which is useful both as an example of a genotype which is respon sive to environmental pressures and as a generic tool for the developm ent of new methodology for the analysis of bacterial communities in na tural environments.