Molecular cloning and sequencing of the hemD gene of Escherichia coli K-12 and preliminary data on the Uro operon

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Abstract
DNA of plasmid pSAS1002TH (F' ilv+ hemD+ hemC+ cya+) was used to clone the hemD gene of Escherichia coli K-12. Due to poor transformability of the heme-deficient mutants, the restriction fragments of the F' plasmid were first cloned into a mobilizable derivative of pBR322, pSAS1211LP, which was then mobilized into a hemD recA mutant (E. coli SASX419AN). One recombinant plasmid, carrying a HindIII fragment of about 5 kilobases (kb), was shown to complement the hemD mutant and also a cya mutant of E. coli K-12, as well as a hemC mutant of Salmonella typhimurium LT2. Further subcloning of the insert enabled us to locate the hemD gene to a BamHI-PstI fragment (approximately 2.3 kb) which also carried the hemC gene. The hemD gene occupies a region close to the PstI end, since the deletion of a 0.6-kb fragment from this end resulted in loss of the ability to complement the hemD mutation. The use of the promoter-probe vector pK01 and the results of complementation showed that the hemD gene was transcribed under physiological conditions from the same promoter as the hemC gene, the direction of transcription being hemC-hemD. This allows us to define a new polycistronic operon of E. coli K-12, for which we propose the designation Uro operon. Sequencing of the hemD gene showed the presence of an open reading frame (ORF) of 738 nucleotides which could code for a protein with a molecular weight of 27,766, which should correspond to the hemD protein; the ORF starts with the last nucleotide of the hemC gene, the two genes having different reading frames. An ORF of at least 480 base pairs follows the hemD gene after a few nucleotides. The corresponding gene X, the function of which is unknown, might represent a third member of the Uro operon.