Regulation of heme synthesis by iron

We devised a screen for isolating mutants defective in iron-dependent regulation of heme biosynthesis that did not require prior knowledge of the mechanism or of the rate-limiting steps [83]. We speculated that if the pathway as a whole were regulated by iron, a mutant defective in that control would accumulate protoporphyrin under iron limitation. Mutants defective in the heme synthesis enzymes ferrochelatase [75] or protoporphyrinogen oxidase would likely have a similar phenotype, but porphyrin accumulation would likely be independent of iron in the structural gene mutants, and those strains would also be expected to be heme auxotrophs. 

Protoporphyrin is a fluorescent compound whereas heme is not, and therefore we screened a population of Tn5-induced mutants that formed fluorescent colonies imder ultraviolet light. The resultant mutant, strain LODTM5, had the desired phenotype in that it accumulated protoporphyrin only under iron limitation, it was not defective in the last two steps of the heme pathway, and it was not a heme auxotroph. Strain LODTM5 carries a loss-of-function mutation in a new gene named irr (iron responsive regulator) that encodes a protein predicted to contain a helix-tum-helix motif of the GntR family of bacterial transcriptional regulators. 

The irr gene is regulated by iron, resulting in expression only in cells grown under iron limitation. We found that hemB, which is normally regulated by iron, is con- 

1 Iron-dependent regulation of bacterial heme biosynthesis 

A search of the databases reveals that B. japonicum Irr shares the greatest homology with Fur from Pseudomonas aeruginosa, showing 29% identity. Despite this structural homology, it is clear that Irr is not a functional Fur homolog. Firstly, Fur requires iron for activity and therefore it functions in iron replete cells. By contrast. 

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