IDENTIFICATION AND MOLECULAR-GENETIC ANALYSIS OF MULTIPLE LOCI CONTRIBUTING TO HIGH-LEVEL TELLURITE RESISTANCE IN RHODOBACTER-SPHAEROIDES-2.4.1

The ability of the facultative photoheterotroph Rhodobacter sphaeroide s to tolerate and reduce high levels of tellurite in addition to at le ast 10 other rare earth metal oxides and oxyanions has considerable po tential for detoxification and bioremediation of contaminated environm ents. We report the identification and characterization of two loci in volved in high-level tellurite resistance. The first locus contains fo ur genes, two of which, trgAB, confer increased tellurite resistance w hen introduced into the related bacterium Paracoccus denitrificans. Th e trgAB-derived products display no significant homology to known prot eins, but both are likely to be membrane-associated proteins. Immediat ely downstream of trgB, the cysK (cysteine synthase) and orf323 genes were identified. Disruption of the cysK gene resulted in decreased tel lurite resistance in R. sphaeroides, confirming earlier observations o n the importance of cysteine metabolism for high-level tellurite resis tance. The second locus identified is represented by the telA gene, wh ich is separated from trgAB by 115 kb. The telA gene product is 65% si milar to the product of the klaB (telA) gene from the tellurite-resist ance-encoding kilA operon from plasmid RK2. The genes immediately link ed to the R. sphaeroides telA gene have no similarity to other compone nts of the kilA operon. R. sphaeroides telA could not functionally sub stitute for the plasmid RK2 telA gene, indicating substantial function al divergence between the two gene products. However, inactivation of R. sphaeroides telA resulted in a significant decrease in tellurite re sistance compared to the wild-type strain. Both cysK and telA null mut ations readily gave rise to suppressors, suggesting that the phenomeno n of high-level tellurite resistance in R. sphaeroides is complex and other, as yet uncharacterized, loci may be involved.