Cloning and Sequence Analysis of the Mercury Resistance Operon of Streptomyces sp. Strain CHR28 Reveals a Novel Putative Second Regulatory Gene

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Abstract
A DNA library of pRJ28, a large linear plasmid encoding mercury resistance, was constructed, and the mercury resistance genes were cloned. The 5,921-bp sequence was analyzed and showed a high degree of similarity to the Streptomyces lividans 1326 mercury resistance operon. Genes merR , merT , merP , and orfIV were found in a similar order and in a single transcription unit. merA and merB were found to be transcribed in the opposite direction to genes merR , merT , merP , and orfIV , as in S. lividans 1326. A novel putative regulatory gene, orfX , was found 22 bp downstream of merA. orfX encodes a 137-amino acid protein with a potential helix-turn-helix motif in the N-terminal domain, characteristic of the MerR family of transcriptional regulators. Transcriptional studies showed that orfX is cotranscribed with merA and merB . It is hypothesized that orfX plays a role in the regulation of the mercury resistance operon, probably by binding at the MerR operator site.