Protoporphyrinogen oxidase (Protox) catalyses the oxidation of protoporphyrinogen IX to protoporphyrin IX during the synthesis of tetrapyrrole molecules. Protox is encoded by the hemY gene in eukaryotes and by the hemG gene in many γ-proteobacteria, including Escherichia coli. It has been suggested that other bacteria possess a yet unidentified type of Protox. To identify a unique bacterial gene encoding Protox, we first introduced the Arabidopsis hemY gene into the genome of the cyanobacterium, Synechocystis sp. PCC6803. We subsequently mutagenized the cells by transposon tagging and screened the tagged lines for mutants that were sensitive to acifluorfen, which is a specific inhibitor of the hemY-type Protox. Several cell lines containing the tagged slr1790 locus exhibited acifluorfen sensitivity. The slr1790 gene encodes a putative membrane-spanning protein that is distantly related to the M subunit of NADH dehydrogenase complex I. We attempted to disrupt this gene in the wild-type background of Synechocystis, but we were only able to obtain heteroplasmic disruptants. These cells accumulated a substantial amount of protoporphyrin IX, suggesting that the slr1790 gene is essential for growth and Protox activity of cells. We found that most cyanobacteria and many other bacteria possess slr1790 homologs. We overexpressed an slr1790 homolog of Rhodobacter sphaeroides in Escherichia coli and found that this recombinant protein possesses Protox activity in vitro. These results collectively demonstrate that slr1790 encodes a unique Protox enzyme and we propose naming the slr1790 gene “hemJ.

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