Thiobacillus ferrooxidans mer operon: sequence analysis of the promoter and adjacent genes

The merA upstream nucleotide (nt) sequence (1378 bp) of the Hg²⁺-resistance-encoding gene system (mer) in Thiobacillus ferrooxidans, was determined. The region contains two open reading frames: unidentified reading frame 1 (URF1) and merC. URF1 has 63-73% homology with those of Tn501, R100 and pDU1358, although the corresponding product has not yet been identified. Thiobacillus merC had 61% and 55% homology with R100 merC at the nt and amino acid (aa) sequence levels, respectively, and its product, consisting of 143 aa, was highly hydrophobic. No sequences homologous to merR, merT, merP or merD of R100 were observed on either strand. Within the 70-100-bp sequence upstream from the merC start codon, there was a sequence highly homologous to the promoter of merT of other Gram^- mer systems. From primer extension and Northern-blot analyses, it became clear that merC and merA were co-transcribed from this putative transcription start point. The mer transcript in T. ferrooxidans was only detected in Hg²⁺-induced cells. Therefore, it was concluded that the T. ferrooxidans mer system is an inducible operon.