Cadmium triggers an integrated reprogramming of the metabolism of Synechocystis PCC6803, under the control of the Slr1738 regulator.

BACKGROUND: Cadmium is a persistent pollutant that threatens most biological organisms, including cyanobacteria that support a large part of the biosphere. Using a multifaceted approach, we have investigated the global responses to Cd and other relevant stresses (H2O2 and Fe) in the model cyanobacterium Synechocystis PCC6803. RESULTS: We found that cells respond to the Cd stress in a two main temporal phases process. In the "early" phase cells mainly limit Cd entry through the negative and positive regulation of numerous genes operating in metal uptake and export, respectively. As time proceeds, the number of responsive genes increases. In this "massive" phase, Cd downregulates most genes operating in (i) photosynthesis (PS) that normally provides ATP and NADPH; (ii) assimilation of carbon, nitrogen and sulfur that requires ATP and NAD(P)H; and (iii) translation machinery, a major consumer of ATP and nutrients. Simultaneously, many genes are upregulated, such as those involved in Fe acquisition, stress tolerance, and protein degradation (crucial to nutrients recycling). The most striking common effect of Cd and H2O2 is the disturbance of both light tolerance and Fe homeostasis, which appeared to be interdependent. Our results indicate that cells challenged with H2O2 or Cd use different strategies for the same purpose of supplying Fe atoms to Fe-requiring metalloenzymes and the SUF machinery, which synthesizes or repairs Fe-S centers. Cd-stressed cells preferentially breakdown their Fe-rich PS machinery, whereas H2O2-challenged cells preferentially accelerate the intake of Fe atoms from the medium. CONCLUSION: We view the responses to Cd as an integrated "Yin Yang" reprogramming of the whole metabolism, we found to be controlled by the Slr1738 regulator. As the Yin process, the ATP- and nutrients-sparing downregulation of anabolism limits the poisoning incorporation of Cd into metalloenzymes. As the compensatory Yang process, the PS breakdown liberates nutrient assimilates for the synthesis of Cd-tolerance proteins, among which we found the Slr0946 arsenate reductase enzyme.

Arsenic and selenium in microbial metabolism.

Arsenic and selenium are readily metabolized by prokaryotes, participating in a full range of metabolic functions including assimilation, methylation, detoxification, and anaerobic respiration. Arsenic speciation and mobility is affected by microbes through oxidation/reduction reactions as part of resistance and respiratory processes. A robust arsenic cycle has been demonstrated in diverse environments. Respiratory arsenate reductases, arsenic methyltransferases, and new components in arsenic resistance have been recently described. The requirement for selenium stems primarily from its incorporation into selenocysteine and its function in selenoenzymes. Selenium oxyanions can serve as an electron acceptor in anaerobic respiration, forming distinct nanoparticles of elemental selenium that may be enriched in (76)Se. The biogenesis of selenoproteins has been elucidated, and selenium methyltransferases and a respiratory selenate reductase have also been described. This review highlights recent advances in ecology, biochemistry, and molecular biology and provides a prelude to the impact of genomics studies.