NUCLEOTIDE-SEQUENCE OF THE HEMG GENE INVOLVED IN THE PROTOPORPHYRINOGEN OXIDASE ACTIVITY OF ESCHERICHIA-COLI K12

The hemG gene of Escherichia coli K12 is involved in the activity of p rotoporphyrinogen oxidase, the enzyme responsible for the conversion o f protoporphyrinogen IX into protoporphyrin IX during heme and chlorop hyll biosynthesis. The gene is located at min 87 on the genetic map of E. coli K12. The hemG gene was isolated by a mini-Mu in vivo cloning procedure. As expected, the hemG gene is able to restore normal growth to the hemG mutant, and the transformed cells display strong protopor phyrinogen oxidase activity. Sequencing of the heme gene allowed us to identify an open reading frame of 546 nucleotides (181 amino acids), within the minimal fragment able to complement the mutant. The presume d molecular mass of the HemG protein is 21 202 Da, in agreement with v alues found by SDS-PAGE, in a DNA-directed coupled transcription-trans lation system. The identity of the first 18 amino acids at the amino-t erminal end of the protein was confirmed by microsequencing. To our kn owledge, this is the first cloning of a gene involved in the protoporp hyrinogen oxidase activity of E. coli.