The expression of the superoxide dismutase gene in Halobacterium cutirubrum and Halobacterium volcanii

The gene encoding the Mn-containing superoxide dismutase (SOD) from Halobacterium cutirubrum has been cloned and sequenced. The deduced amino acid sequence is homologous to the sequences of Fe and Mn SODs from eubacteria. The high degree of amino acid identity between the archaebacterial and eubacterial proteins suggests that a SOD gene may have been laterally transferred between eubacteria and archaebacteria sometime after the accumulation of atmospheric oxygen. Consensus elements of halobacterial promoters are found upstream of the coding region, however, the spacing between them and the transcription start site is greater than in other genes. Termination of transcription occurs in five consecutive T residues that are preceded by a GC-rich sequence that has short inverted repeats. In addition to the authentic SOD gene, H. cutirubrum also contains a putative pseudogene. The SOD levels and growth rates of H. cutirubrum and Halobacterium volcanii were tested in response to treatment by paraquat, an intracellular generator of superoxide. In H. volcanii the growth rate slowed, and SOD was strongly induced throughout prolonged treatment with paraquat. In H. cutirubrum the same effects were noticed initially, but after 48 h exposure to the drug, the growth rate increased and the SOD level decreased. Production of paraquat resistant mutants of H. cutirubrum may play a part in this process, however, some type of physiological adaptation is also probably required.