Molecular Analysis of Two Bacterioferritin Genes, bfr α and bfr β, in the Model Rhizobacterium Pseudomonas putida KT2440

The model rhizobacteriumPseudomonas putida KT2440 and other fluorescent pseudomonads possess two bacterioferritins, Bfrα and Bfrβ. However, the regulatory systems controlling the expression of these genes and the roles of these proteins in iron homeostasis are ill defined. Our studies show that bothbfr α andbfr β were monocistronic: promoter motifs and transcriptional start sites were identified, and Fur boxes and σS -dependent regulatory motifs were absent. The expressions ofbfr α andbfr β were enhanced by iron exposure and were maximal in cells rapidly growing in a high-iron environment. Bothbfr α andbfr β were positively regulated by Fur, and both were expressed independently of adjoining, functionally related genes. The loss of Bfrα or Bfrβ individually resulted in a significant reduction (ca. 17%) in cellular iron levels, and the deletion of bothbfr α andbfr β reduced cellular iron levels by 38% relative to those of the wild type. The mutants varied in their abilities to grow in low-iron medium; while growths (rate and final cell density) of single mutants and the wild type were similar, that of the double mutant was reduced significantly. Mutants lacking Bfrα and/or Bfrβ showed no change relative to the wild type in sensitivity to reactive oxygen species toxicity. Collectively, the data show that while Bfrα and Bfrβ could function independently of each other, an interaction-dependent function cannot be ruled out. Furthermore, regardless of the mechanism, a primary benefit of the bacterioferritins toP. putida KT2440 appears to be the enhancement of its survival in the environment by strengthening its tolerance to iron starvation.