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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 y-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 hemYgene 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 acifluorten, which is a specific inhibitor of the hem V-type Protox. Several cell lines containing the tagged sIr1790 locus exhibited acifluorfen sensitivity. The sIn 790 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 sIn 790 gene is essential for growth and Protox activity of cells. We found that most cyanobacteria and many other bacteria possess sIr1790 homologs. We overexpressed an sIr1790 homolog of Rhodobacter sphaeroides in Eschenichia coli and found that this recombinant protein possesses Protox activity in vitro. These results collectively demonstrate that sIr1790 encodes a unique Protox enzyme and we propose naming the sIr1790 gene "hemJ." [ABSTRACT FROM AUTHOR]