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Perception and response to nutritional iron by bacteria is essential for viability, and for the ability to adapt to the environment. The iron response regulator (Irr) is part of a novel regulatory scheme employed by Rhizobium and other Alpha-Proteobacteria to control iron-dependent gene expression. Bradyrhizobium japonicum senses iron through the status of heme biosynthesis to regulate gene expression, thus it responds to an iron-dependent process rather than to iron directly. Irr mediates this response by interacting directly with ferrochelatase, the enzyme that catalyzes the final step in heme biosynthesis. Irr is expressed under iron limitation to both positively and negatively modulate gene expression, but degrades in response to direct binding to heme in iron-sufficient cells. Studies with Rhizobium reveal that the regulation of iron homeostasis in bacteria is more diverse than has been generally assumed.

Iron utilization by bacteria in aerobic environments involves uptake as a ferric chelate from the environment, followed by reduction to the ferrous form. Ferric iron reduction is poorly understood in most bacterial species. Here, we identified Bradyrhizobium japonicum frcB (bll3557) as a gene adjacent to, and coregulated with, the pyoR gene (blr3555) encoding the outer membrane receptor for transport of a ferric pyoverdine. FrcB is a membrane-bound, diheme protein, characteristic of eukaryotic ferric reductases. Heme was essential for FrcB stability, as were conserved histidine residues in the protein that likely coordinate the heme moieties. Expression of the frcB gene in Escherichia coli conferred ferric reductase activity on those cells. Furthermore, reduced heme in purified FrcB was oxidized by ferric iron in vitro. B. japonicum cells showed inducible ferric reductase activity in iron-limited cells that was diminished in an frcB mutant. Steady-state levels of frcB mRNA were strongly induced under iron-limiting conditions, but transcript levels were low and unresponsive to iron in an irr mutant lacking the global iron response transcriptional regulator Irr. Thus, Irr positively controls the frcB gene. FrcB belongs to a family of previously uncharacterized proteins found in many proteobacteria and some cyanobacteria. This suggests that membrane-bound, heme-containing ferric reductase proteins are not confined to eukaryotes but may be common in bacteria.

Children are aware of microbes from a young age and are rightly encouraged to wash their hands to prevent illness. However, myriad microbes live in, on, and around us, most of which are benign or beneficial. Our goal was to teach elementary students about microbiota by leveraging familiar literacy practices, social studies themes, and the arts to advance students’ knowledge and reasoning skills in science. With this perspective in mind, we developed and implemented an interdisciplinary unit targeted at second grade, in which students learned about microbes and microbial communities. Our goal was to further students’ conceptual knowledge of the microbes that surround them by purposefully integrating microbial communities within the second grade curriculum. Throughout the unit, students engaged in hands-on, inquiry-based science experiences and used multimodal communication (through a combination of linguistic, visual, audio, gestural, and spatial modes): they sampled microbes from their own bodies and/or environments and applied their knowledge and imagination to create their own microbes through art and story-telling, generating a class microbial community—both literal and artistic. At the end of the unit, students demonstrated knowledge of microbes and of the diversity and ubiquity of microbial communities and habitats.