Modulation of Medium pH by Caulobacter crescentus Facilitates Recovery from Uranium-Induced Growth Arrest

Abstract

The oxidized form of uranium [U(VI)] predominates in oxic environments and poses a major threat to ecosystems. Due to its ability to mineralize U(VI), the oligotroph Caulobacter crescentus is an attractive candidate for U(VI) bioremediation. However, the physiological basis for U(VI) tolerance is unclear. Here we demonstrated that U(VI) caused a temporary growth arrest in C. crescentus and three other bacterial species, although the duration of growth arrest was significantly shorter for C. crescentus. During the majority of the growth arrest period, cell morphology was unaltered and DNA replication initiation was inhibited. However, during the transition from growth arrest to exponential phase, cells with shorter stalks were observed, suggesting a decoupling between stalk development and the cell cycle. Upon recovery from growth arrest, C. crescentus proliferated with a growth rate comparable to that of a control without U(VI), although a fraction of these cells appeared filamentous with multiple replication start sites. Normal cell morphology was restored by the end of exponential phase. Cells did not accumulate U(VI) resistance mutations during the prolonged growth arrest, but rather, a reduction in U(VI) toxicity occurred concomitantly with an increase in medium pH. Together, these data suggest that C. crescentus recovers from U(VI)-induced growth arrest by reducing U(VI) toxicity through pH modulation. Our finding represents a unique U(VI) detoxification strategy and provides insight into how microbes cope with U(VI) under nongrowing conditions, a metabolic state that is prevalent in natural environments.

Transposon Mutagenesis Paired with Deep Sequencing of Caulobacter crescentus under Uranium Stress Reveals Genes Essential for Detoxification and Stress Tolerance

“…Results revealed that 10 of the 15 mutants exhibited a growth defect in the presence of U relative to growth of the wild type. We later discovered that the pH of PYE medium decreases from 6.5 to 5.7 with addition of 350 M uranyl nitrate, affecting growth phenotypes (25). To uncouple the U response from changes in pH and growth conditions, we tested the ability of each mutant to survive U exposure under buffered conditions (Fig.…”

“…Transcriptomic and proteomic studies during U exposure in C. crescentus revealed dozens of genes with significant changes in expression in response to U; these genes do not appear to overlap substantially with those that are variably expressed in response to other heavy metal stresses, such as cadmium (Cd) and chromium, suggesting a divergent cellular response to U(VI) (23,24). This response manifests, in part, as a temporary arrest in cell cycle progression and DNA replication, along with some cell filamentation (a possible consequence of U-induced DNA damage) observed during growth recovery following U detoxification (25). Proteins upregulated in response to U include the periplasmic protein UrcA, a phytase enzyme, two-component signaling factors, and an ABC transporter.…”

Identification of a U/Zn/Cu responsive global regulatory two‐component system in Caulobacter crescentus

“…C. crescentus is an obligate aerobe with a ubiquitous presence in aqueous environments where it is well‐adapted to life under low‐nutrient conditions (Poindexter, ). Remarkably, Caulobacter species tolerate high concentrations of U (Hu et al ., ; Park and Jiao, ), have been found in U‐contaminated sites (Bollmann et al ., ) and can mineralize U through the formation of uranyl phosphate precipitates (Yung and Jiao, ). Multi‐omics studies to elucidate the U stress response pathways in C. crescentus have revealed that many of the highest induced genes/proteins are, in general, poorly characterized or of unknown function (Hu et al ., ; Yung et al ., ).…”

Uranium Bio-Transformations: Chemical or Biological Processes?

“…This build up should eventually obstruct normal cellular function and is noted anecdotally, but lacks concrete evidence. Park and Jiao reported that C. crescentus exhibits growth arrest and cessation of DNA replication when first exposed to uranium [67]. In the same study, they describe a novel mechanism in which C. crescentus raises the pH of the solution making U(VI) less soluble and consequently less toxic [67], which differs from other detoxification strategies which involve reduction and precipitation.…”

“…Park and Jiao reported that C. crescentus exhibits growth arrest and cessation of DNA replication when first exposed to uranium [67]. In the same study, they describe a novel mechanism in which C. crescentus raises the pH of the solution making U(VI) less soluble and consequently less toxic [67], which differs from other detoxification strategies which involve reduction and precipitation. Yung et al studying the same species found in uranium-exposed cells an upregulation of an ABC-type transporter interpreted as possibly providing efflux for detoxification [69].…”