Tellurite resistance of Proteus mirabilis.

Proteus mirabilis a Gram-negative urinary tract pathogen, displays a form of developmental behaviour on solid medium, termed swarming, that includes cycles of outward migration interspersed with periods of consolidation. To identify developmentally regulated genes, I used transposon mutagenesis to generate promoter-reporter gene fusions. One of the fusions exhibited negative regulation of the reporter gene expression during swarmer cell differentiation (namely, the short vegetative cells showed four-fold increase in the reporter-gene activity as compared to the swarmer cells), and was studied further by subcIoning flanking DNA fragments into pBR322. The mini-transposon (Tn5-lacZ) insertion was located within an open reading frame (ORF) termed terC, a gene associated with tellurite resistance in enteric bacteria. Further analysis of flanking terC sequences revealed six open reading frames designated terZ, -A, -B, -C, -D and -E. All of the putative proteins encoded by these genes showed extensive amino acid sequence similarity with the previously characterized gene products of the IncH12 tellurite resistance (Te ') operons from plasmids pR478 and pMER610 found in Gram-negative bacteria. A screen of 31 clinical isolates of P. mirabilis and 4 strains of Proteus vulgaris indicated that resistance is a common feature of this genus. Insertion of the mini-transposon into terC reduced the level of tellurite resistance by more than 50%. The pR478-derived ter locus restored wild type levels of tellurite resistance in the mutant strain. Northern blot analysis revealed several transcripts that were detectable only when the wild type bacteria were grown in the presence of tellurite. The 5 kb transcript was disrupted by the mini transposon insertion in the terC of the mutant. These studies suggest that the ter locus may be negatively regulated during swarmer cell development, but is positively regulated by the presence of tellurite. This thesis further suggests that Proteus mirabilis may be the evolutionary origin of the plasmid-borne ter loci.Thesis (Ph.D.)--Dalhousie University (Canada), 1999

Sulfate Assimilation Mediates Tellurite Reduction and Toxicity in Saccharomyces cerevisiae▿†

Despite a century of research and increasing environmental and human health concerns, the mechanistic basis of the toxicity of derivatives of the metalloid tellurium, Te, in particular the oxyanion tellurite, Te(IV), remains unsolved. Here, we provide an unbiased view of the mechanisms of tellurium metabolism in the yeast Saccharomyces cerevisiae by measuring deviations in Te-related traits of a complete collection of gene knockout mutants. Reduction of Te(IV) and intracellular accumulation as metallic tellurium strongly correlated with loss of cellular fitness, suggesting that Te(IV) reduction and toxicity are causally linked. The sulfate assimilation pathway upstream of Met17, in particular, the sulfite reductase and its cofactor siroheme, was shown to be central to tellurite toxicity and its reduction to elemental tellurium. Gene knockout mutants with altered Te(IV) tolerance also showed a similar deviation in tolerance to both selenite and, interestingly, selenomethionine, suggesting that the toxicity of these agents stems from a common mechanism. We also show that Te(IV) reduction and toxicity in yeast is partially mediated via a mitochondrial respiratory mechanism that does not encompass the generation of substantial oxidative stress. The results reported here represent a robust base from which to attack the mechanistic details of Te(IV) toxicity and reduction in a eukaryotic organism