We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared.

Microbial heavy-metal resistance

What makes a heavy metal resistant bacterium heavy metal resistant? The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance–nodulation–cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors. The complement of efflux systems of 63 sequenced prokaryotes was compared with that of the heavy metal resistant bacterium Ralstonia metallidurans. This comparison shows that heavy metal resistance is the result of multiple layers of resistance systems with overlapping substrate specificities, but unique functions. Some of these systems are widespread and serve in the basic defense of the cell against superfluous heavy metals, but some are highly specialized and occur only in a few bacteria. Possession of the latter systems makes a bacterium heavy metal resistant.

Efflux‐mediated heavy metal resistance in prokaryotes

All aerobically growing organisms suffer exposure to oxidative stress, caused by partially reduced forms of molecular oxygen, known as reactive oxygen species (ROS). These are highly reactive and capable of damaging cellular constituents such as DNA, lipids and proteins. Consequently, cells from many different organisms have evolved mechanisms to protect their components against ROS. This review concentrates on the oxidant defence systems of the budding yeast Saccharomyces cerevisiae, which appears to have a number of inducible adaptive stress responses to oxidants, such as H2O2, superoxide anion and lipid peroxidation products. The oxidative stress responses appear to be regulated, at least in part, at the level of transcription and there is considerable overlap between them and many diverse stress responses, allowing the yeast cell to integrate its response towards environmental stress.

/pdf/oxidative-stress-responses-of-the-yeast-saccharomyces-1tg2xtod2r.pdf

Oxidative stress responses of the yeast Saccharomyces cerevisiae.

The last ten years have witnessed major advances in our understanding of zinc transporters and their regulation in eukaryotic organisms. Two families of transporters, the ZIP (Zrt-, Irt-like Protein) and CDF (Cation Diffusion Facilitator) families, have been found to play a number of important roles in zinc transport. These are ancient gene families that span all phylogenetic levels. The characterized members of each group have been implicated in the transport of metal ions, frequently zinc, across lipid bilayer membranes. This remarkable conservation of function suggests that other, as yet uncharacterized members of the family, will also be involved in metal ion transport. Many of the ZIP family transporters are involved in cellular zinc uptake and at least one member, the Zrt3 transporter of S. cerevisiae, transports stored zinc out of an intracellular compartment during adaptation to zinc deficiency. In contrast, CDF family members mediate zinc efflux out of cells or facilitate zinc transport into intracellular compartments for detoxification and/or storage. The activity of many of these transporters is regulated in response to zinc through transcriptional and post-transcriptional mechanisms to maintain zinc homeostasis at both the cellular and organismal levels.

Eukaryotic zinc transporters and their regulation.

▪ Abstract Transition metals such as iron, copper, manganese, and zinc are essential nutrients. The yeast Saccharomyces cerevisiae is an ideal organism for deciphering the mechanism and regulation of metal ion transport. Recent studies of yeast have shown that accumulation of any single metal ion is mediated by two or more substrate-specific transport systems. High-affinity systems are active in metal-limited cells, whereas low-affinity systems play the predominant roles when the substrate is more abundant. Metal ion uptake systems of cells are tightly controlled, and both transcriptional and posttranscriptional regulatory mechanisms have been identified. Most importantly, studies of S. cerevisiae have identified a large number of genes that function in metal ion transport and have illuminated the existence and importance of gene families that play related roles in these processes in mammals.

The molecular biology of metal ion transport in Saccharomyces cerevisiae

The OSR (oxidative-stress-resistance) gene was cloned as the gene which enhances resistance against oxidative stress caused by lipid hydroperoxide in Saccharomyces cerevisiae. The nucleotide sequence of the gene was determined and found to be identical with the zinc-resistance-conferring (ZRCI) gene, which was cloned as the gene enhancing the resistance against zinc and cadmium. A knockout mutant of the OSR gene showed increased sensitivity to lipid hydroperoxide. The intracellular glutathione content in the knockout mutant decreased approx. 40% compared with that of wild-type cells, whereas it increased 3-fold in yeast overexpressing the OSR gene. Therefore the OSR/ZRCI gene product seems to regulate the intracellular glutathione content.

Correlation of the OSR/ZRCI gene product and the intracellular glutathione levels in Saccharomyces cerevisiae.

Two kinds of DNA fragments concerned with resistance against lipid hydroperoxide were cloned from the yeast Saccharomyces cerevisiae. When the recombinant plasmid carrying the DNA fragment was introduced into the cells of S. cerevisiae which do not have an appropriate auxotrophic marker, the transformants could be selected by their resistance to t-butyl hydroperoxide. By Southern hybridization analysis, such transformants were found to contain the recombinant plasmid, suggesting that the DNA fragment could be used as a selection marker for the transformation of S. cerevisiae.

Lipid hydroperoxide-resistance gene in Saccharomyces cerevisiae : utilization as a selectable marker gene for yeast transformation

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Correlation of the OSR/ZRCI gene product and the intracellular glutathione levels in Saccharomyces cerevisiae.

Lipid hydroperoxide-resistance gene in Saccharomyces cerevisiae : utilization as a selectable marker gene for yeast transformation