Antibiotic resistance in microorganisms is now worldwide, in large part related to the spread of plasmids and acquisition of transposable resistances. Among many bacteria, however, newly discovered resistance determinants on the chromosome have emerged, generally to one, but occasionally to more than one group of antibiotics. We have recently described in E. coli a chromsomally-mediated amplifiable high-level resistance to six structurally different antibiotics with different target sites in mutants selected by growth in one drug. Several genes are involved in this """"""""multiple antibiotic resistance"""""""" (Mar) system; however, inactivation of one genetic locus, designated marA, results in complete loss of all resistances. The finding of cryptic chromosmal genes which express resistance and which have appeared and may emerge under clinical situations is of concern, particularly since we already know that the resistance mechanisms are at least in some cases distinct from those found on plasmids. Our proposed studies are aimed at understanding this new genetic system and these new mechanisms of resistance before they emerge as clinical problems. In addition, the bacterial Mar system has several striking characteristics in common with the amplifiable, multiresistance found in parasites and human and mammalian cancer cells in response to treatment with single chemotherapeutic drugs. Research on Mar may help to illuminate this general biologic response of cells to growth inhibitory agents. We shall use biochemical, molecular and genetic studies to examine the genes, gene products, and mechanisms for the multiple resistance phenotypes seen in the Mar mutants of E. coli. Such studies will involve 1) mapping and cloning the genes involved, 2) determining the genetic basis of the stepwise selection leading to high level resistance, 3) sequencing the marA locus, 4) sequencing some of the other genes involved in resistance, and 5) determining gene products and the biochemical and functional bases of the resistances. In order to gain insight into possible evolutionary links, we shall compare Mar-related chromosomal to resistance to those which we have already characterized on plasmids.
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