The detection and characterization of genetically programmed responses to environmental stress

Microbial reporters of metal bioavailability

When attempting to assess the extent and the implications of environmental pollution, it is often essential to quantify not only the total concentration of the studied contaminant but also its bioavailable fraction: higher bioavailability, often correlated with increased mobility, signifies enhanced risk but may also facilitate bioremediation. Genetically engineered microorganisms, tailored to respond by a quantifiable signal to the presence of the target chemical(s), may serve as powerful tools for bioavailability assessment. This review summarizes the current knowledge on such microbial bioreporters designed to assay metal bioavailability. Numerous bacterial metal‐sensor strains have been developed over the past 15 years, displaying very high detection sensitivities for a broad spectrum of environmentally significant metal targets. These constructs are based on the use of a relatively small number of gene promoters as the sensing elements, and an even smaller selection of molecular reporter systems; they comprise a potentially useful panel of tools for simple and cost‐effective determination of the bioavailability of heavy metals in the environment, and for the quantification of the non‐bioavailable fraction of the pollutant. In spite of their inherent advantages, however, these tools have not yet been put to actual use in the evaluation of metal bioavailability in a real environmental remediation scheme. For this to happen, acceptance by regulatory authorities is essential, as is a standardization of assay conditions.

Cadmium-regulated gene fusions in Pseudomonas fluorescens

To study the mechanisms soil bacteria use to cope with elevated concentrations of heavy metals in the environment, a mutagenesis with the lacZ-based reporter gene transposon Tn5B20 was performed. Random gene fusions in the genome of the common soil bacterium Pseudomonas fluorescens strain ATCC 13525 were used to create a bank of 5,000 P. fluorescens mutants. This mutant bank was screened for differential gene expression in the presence of the toxic metal cadmium. Fourteen mutants were identified that responded with increased or reduced gene expression to the presence of cadmium. The mutants were characterized with respect to their metal-dependent gene expression and their metal tolerance. Half the identified mutants reacted with differential gene expression specifically to the metal cadmium, whereas some of the other mutants also responded to elevated concentrations of copper and zinc ions. One of the mutants, strain C8, also showed increased gene expression in the presence of the solvent ethanol, but otherwise no overlap between cadmium-induced gene expression and general stress response was detected. Molecular analysis of the corresponding genetic loci was performed using arbitrary polymerase chain reaction (PCR), DNA sequencing and comparison of the deduced protein products with sequences deposited in genetic databases. Some of the genetic loci targeted by the transposon did not show any similarities to any known genes; thus, they may represent 'novel' loci. The hypothesis that genes that are differentially expressed in the presence of heavy metals play a role in metal tolerance was verified for one of the mutants. This mutant, strain C11, was hypersensitive to cadmium and zinc ions. In mutant C11, the transposon had inserted into a genetic region displaying similarity to genes encoding the sensor/regulator protein pairs of two-component systems that regulate gene expression in metal-resistant bacteria, including czcRS of Ralstonia eutropha, czrRS of Pseudomonas aeruginosa and copRS of Pseudomonas syringae. Although the P. fluorescens strain used in this study had not been isolated from a metal-rich environment, it nevertheless contained at least one genetic region enabling it to cope with elevated concentrations of heavy metals.