Evolution and regulation of the gene encoding superoxide dismutase from the archaebacterium Halobacterium cutirubrum

The gene encoding the manganese-containing superoxide dismutase (SOD) of Halobacterium cutirubrum was isolated and characterized. The gene and 5'- and 3'-untranslated regions were located on a genomic DNA fragment of 1127 nucleotides. The deduced amino acid sequence is 200 residues long and has 39-42% identity with manganese-containing SODs of eubacteria and mitochondria. This homology may be due to either lateral transfer of the gene between eubacteria and archaebacteria or to high amino acid sequence conservation in the enzyme during the separate evolution of eubacteria and archaebacteria. Transcription of the gene initiates only about three nucleotides upstream of the translation initiation codon. The 5' end of the transcript does not contain a purine-rich Shine-Dalgarno sequence, and the promoter region does not contain consensus sequences found in other archaebacterial promoters. Termination of transcription occurs at 5 consecutive thymine residues that are preceded by a GC-rich region. The gene is basally expressed in anaerobically grown cells but is also inducible by paraquat, a generator of oxygen radicals. The same transcription initiation site is used in both types of expression, suggesting that one promoter is responsible for both basal and regulated expression. In addition to the single copy of the authentic SOD gene, the genome of H. cutirubrum contains a sequence that is very closely related to but does not code for the previously purified SOD of this organism.