constraints can be incorporated into cells and used as an in vivo 

 indicator of iron deficiency. 



Reporter genes have been used to detect in situ activity of 

 microbes a few times. New developments in molecular genetics allow 

 us to measure the transcriptional activity of individual genes in 

 situ . While the product of many environmentally important genes 

 may be difficult to assess directly or indirectly in situ due to 

 their low level of expression, such genes can be fused with genes 

 whose products are easily measured in situ . The promoterless 

 "reporter gene" will be expressed only when the environmental gene 

 is transcriptionally active. A promoterless, ice- nucleation gene 

 was recently shown to be a sensitive reporter of the 

 transcriptional activity of iron-regulated Pseudomonas genes that 

 are involved in siderophore-mediated iron uptake in situ in soil. 

 Soil does not contain ice nuclei, and the transcriptional activity 

 of the iron-sensing gene in root-colonizing Pseudomonas stomis 

 could be easily quantified in a simple, droplet-freezing assay that 

 measures the number of ice nuclei produced by containing the 

 reporter-gene strains introduced into the soil. These studies 

 showed the average level of Fe 3+ sensed by bacteria on roots, and 

 demonstrated the heterogeneity of this resource in situ . Such an 

 approach can be immediately applied to all culturable organisms 

 which can be genetically manipulated. Many resource-responsive 

 genes already have been identified that can be fused with 

 appropriate reporter genes to indicate what levels of these 

 resources are sensed by the organism in situ . Organisms containing 

 reporter genes can be introduced into different habitats and used 

 as sensitive "biological sensors" of the transcriptional status of 

 other pertinent genes. Further advances in the ability to 

 genetically manipulate many microbes will be needed, but the 

 benefits from using such genetic tools should justify this effort. 



Tolerance and Sensitivity to Trace Metals 



Toxic trace metals from several different industrial and 

 energetic processes contaminate soil and water. The ability of 

 plants to grow in soils containing normally toxic concentrations of 

 these ions has been known for some time, but the molecular and 

 biochemical mechanisms of metal tolerance are not well understood. 

 Oceanic phytoplankton are extremely sensitive to certain trace 

 metals, while coastal species are relatively tolerant. 



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