Lawrence Berkeley National Laboratory

The bacterium Caulobacter crescentus and related stalk bacterial species are known for their distinctive ability to live in low nutrient environments, a characteristic of most heavy metal contaminated sites. Caulobacter crescentus is a model organism for studying cell cycle regulation with well developed genetics. We have identified the pathways responding to heavy metal toxicity in C. crescentus to provide insights for possible application of Caulobacter to environmental restoration. We exposed C. crescentus cells to four heavy metals (chromium, cadmium, selenium and uranium) and analyzed genome wide transcriptional activities post exposure using a Affymetrix GeneChip microarray. C. crescentus showed surprisingly high tolerance to uranium, a possible mechanism for which may be formation of extracellular calcium-uranium-phosphate precipitates. The principal response to these metals was protection against oxidative stress (up-regulation of manganese-dependent superoxide dismutase, sodA). Glutathione S-transferase, thioredoxin, glutaredoxins and DNA repair enzymes responded most strongly to cadmium and chromate. The cadmium and chromium stress response also focused on reducing the intracellular metal concentration, with multiple efflux pumps employed to remove cadmium while a sulfate transporter was down-regulated to reduce non-specific uptake of chromium. Membrane proteins were also up-regulated in response to most of the metals tested. A two-component signal transduction system involved in the uranium response was identified. Several differentially regulated transcripts from regions previously not known to encode proteins were identified, demonstrating the advantage of evaluating the transcriptome using whole genome microarrays.