Zinc- and arsenic-resistant strains of Thiobacillus ferrooxidans have increased copy numbers of chromosomal resistance genes

1 Research Group of Genosystematics and Evolution of Bacteria, Institute of Microbiology, Russian Academy of Sciences, Moscow 117811, Prospekt 60-let Octyabrya, 7/2, Russia 2 Laboratory of Microbial Transformation of Minerals, Institute of Microbiology, Russian Academy of Sciences, Moscow 117811, Prospekt 60-let Octyabrya, 7/2, Russia 5 Author for correspondence: Tamara F. Kondratyeva. Tel: +7 95 135 65 96. ABSTRACT Pulsed-field gel electrophoresis (PFGE) was used to examine chromosomal DNA from various strains of Thiobacillus ferrooxidans: these were the reference strain VKM-458, strains isolated from different environments and pilot plants for processing gold-bearing concentrates, and strains experimentally adapted to high zinc and arsenic concentrations in growth medium. The restriction endonuclease Xbal digested T. ferrooxidans VKM-458 chromosomal DNA into a number of fragments sufficient for identification of their size and calculation of the size of the entire genome (2855/pm44 kb). Restriction fragment length polymorphism of the chromosomal DNA in various strains suggests the usefulness of this approach for analysis of the diversity of T. ferrooxidans strains and for the study of strain stability under conditions of industrial utilization. A comparison of Xbal -restriction patterns in parent strains and in strains with acquired enhanced resistance to zinc or arsenic revealed amplification of certain fragments in the resistant strains, i.e. a 98 kb fragment in strain TFZ and a 28 kb fragment in strain VKM-458As2. We suggest that the enhanced resistance to toxic metals in T. ferrooxidans is gained through increase of the copy number of resistance genes and enhanced synthesis of proteins involved in resistance. Keywords: pulsed-field gel electrophoresis (PFGE), restriction patterns, restriction fragment length polymorphism (RFLP), arsenic resistance, zinc resistance