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A single superoxide dismutase (Sod) gene was identified in Borrelia burgdorferi strains, Borrelia afzelii Ple and Borrelia garinii Pbi. Recombinant enzymatic activity was detected only when sod expression was controlled by the lacZ promoter in the cloning vector. Northern blot analysis with sod- or secA-specific probes identified a common 3.7-kb transcript. Reverse transcriptase-PCR analysis confirmed that secA and sod constitute a single transcriptional unit in B. burgdorferi. A transcriptional start site of this operon, containing -10 and -35 regions of a sigma(70)-type promoter, was mapped to 100 bp upstream of the ATG start codon of secA.