REGULATION OF THE OPERON RESPONSIBLE FOR BROAD-SPECTRUM MERCURY RESISTANCE IN STREPTOMYCES-LIVIDANS-1326

The broad-spectrum mercury resistance of Streptomyces lividans 1326 is mediated by six open reading frames (orf). These are arranged in two divergently transcribed operons. The orfs merA (mercuric reductase) an d merB (organolyase) form one of the two operons. These genes and thei r regulation were further studied by deletion analysis and transcripti onal fusion to the reporter gene xylE in the plasmid pXE4. An increase in Xy1E activity in response to the presence of mercuric ions was obs erved. The function of ORF2 (MerT) and ORF3 (MerP) as mercury-specific transport proteins, previously postulated based on the structural fea tures of the predicted proteins, was confirmed. Transcription of the m er genes starts within the intercistronic region and two divergent pro moters were identified by S1 nuclease mapping. Expression of the genes was negatively regulated by the product of orf1, now called merR. The repressor function was confirmed by gel retardation assays. MerR, pro duced in Escherichia coli, bound to two sites (operators) in the fragm ent containing the promoter region between merA and merR. Addition of mercuric ions and phenylmercuric acetate prevented the binding of MerR .