Cloning and sequence analysis of the mercury resistance operon of Streptomyces sp strain CHR28 reveals a novel putative second regulatory gene

A DNA library of pRJ28, a large linear plasmid encoding mercury resistance, was constructed, and the mercury resistance genes were cloned. The 5,921-b p sequence was analyzed and showed a high degree of similarity to the Strep tomyces lividans 1326 mercury resistance operon, Genes merR, merT, merP, an d orfIV were found in a similar order and in a single transcription unit. m erA and merB were found to be transcribed in the opposite direction to gene s merR, merT, merP, and orfIV, as in S. lividans 1326. A novel putative reg ulatory 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-termina l domain, characteristic of the MerR family of transcriptional regulators. Transcriptional studies showed that orfX is cotranscribed with merA and mer B. It is hypothesized that orfX plays a role in the regulation of the mercu ry resistance operon, probably by binding at the MerR operator site.