A sensor responsive to iron was constructed by fusing a promoterless ice nucleation activity gene (inaZ) to an iron-regulated promoter of a genomic region involved in pyoverdine (fluorescent siderophore) (pvd) production in Pseudomonas syringae. Cells of Pseudomonas fluorescens and P. syringae that contained the pvd-inaZ fusion expressed iron-responsive ice nucleation activity in the bean rhizosphere and phyllosphere, respectively, and in culture. Addition of Fe(III) to leaves or soil reduced the apparent transcription of the pvd-inaZ reporter gene, as shown by a reduction in the number of ice nuclei produced, indicating that Fe(III) was primarily responsible for mediating transcription of the pvd-inaZ gene even in natural environments. A Pseudomonas sp. strain having an intact iceC gene, which conferred Fe-insensitive expression of ice nucleation activity, was included in all studies to account for small strain- or environment-dependent differences in the ability of bacterial cells to produce ice nuclei. Thus, a comparison of the ice nucleation activity conferred by pvd-inaZ with the activity conferred by iceC revealed the bioavailability of iron in culture or natural habitats. The relative ice nucleation activities expressed by strains containing iceC or pvd-inaZ indicated that, while not abundant, Fe(III) is not present at extremely low concentrations at all microsites colonized by bacteria on plant surfaces. Biological sensors that are constructed by fusing inaZ to chemically responsive promoters provide a novel way to characterize chemical constituents of microbial habitats
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