A BIOLOGICAL SENSOR FOR IRON AVAILABLE TO BACTERIA IN THEIR HABITATS ON PLANT-SURFACES

A sensor responsive to iron was constructed by fusing a promoterless i ce 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 a nd P. syringae that contained the pvd-inaZ fusion expressed iron-respo nsive 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 s hown by a reduction in the number of ice nuclei produced, indicating t hat Fe(III) was primarily responsible for mediating transcription of t he pvd-inaZ gene even in natural environments. A Pseudomonas sp. strai n having an intact iceC gene, which conferred Fe-insensitive expressio n of ice nucleation activity, was included in all studies to account f or small strain- or environment-dependent differences in the ability o f 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 ha bitats. The relative ice nucleation activities expressed by strains co ntaining iceC or pvd-inaZ indicated that, while not abundant, Fe(III) is not present at extremely low concentrations at all microsites colon ized by bacteria on plant surfaces. Biological sensors that are constr ucted by fusing inaZ to chemically responsive promoters provide a nove l way to characterize chemical constituents of microbial habitats.