Transposon Tn5 as an identifiable marker in rhizobia: Survival and genetic stability of Tn5 mutant bean rhizobia under temperature stressed conditions in desert soils

Five transposon Tn5 insertion mutants of a beanRhizobium strain (Rhizobium leguminosarum b. v.phaseoli) were used in an ecological study to evaluate the extent to which transposon Tn5 was stable to serve as an identifiable marker in rhizobia under a high temperature stress condition in two Sonoran Desert soils. All the mutants possessed single chromosomal insertions of the transposon. In both soils, under the temperature stress conditions that were employed (40°C), both wild type and mutant populations possessing functional transposable elements declined rapidly. After 12 days, mutant cells, when screened using the Tn5 coded antibiotic resistance markers, were significantly less in number than when they were screened using only their intrinsic antibiotic resistance markers. There were no significant differences in numbers between the mutant cell population and the wild type when the mutant cells were screened using only the intrinsic antibiotic resistance markers. DNA-DNA hybridizations using a probe indicated neither deletion nor transposition of the transposable element. The results indicate that transposon DNA sequences are present within cells under high temperature stress conditions, but kanamycin/neomycin resistance is not expressed by some of these cells, suggesting that Tn5 undergoes a possible functional inactivation under these conditions. The possible implications of these findings are discussed.