Interactions between Two MerR Regulators and Three Operator/Promoter Regions in the Mercury Resistance Module ofBacillus megaterium

The mercury resistance module of Bacillus transposon TnMERI1 is regulated by three operator/promoter regions (O/P merB3, O/P merR1, and O/P merR2) and two regulatory proteins (MerR1 and MerR2) encoded by the module itself. To clarify the roles of MerR1 and MerR2 in the regulatory mechanism, both proteins were overexpressed and purified. MerR1 bound the regulatory regions O/P merB3 and O/P merR1, with a preference for O/P merB3 as measured on in vitro gel shift assays. However, MerR2 bound O/P merR2, as revealed by gel shift and restriction endonuclease protection assays. The transcriptional start sites of O/P merB3 and O/P merR2 were determined by rapid amplification of 5'-cDNA ends (5'-RACE) in the TnMERI1 original host, Bacillus megaterium strain MB1. Real-time reverse transcription polymerase chain reaction (RT-PCR) assays showed that O/P merB3 and O/P merR1 were induced in the presence of Hg2+ but not O/P merR2. It was concluded that MerR1 regulates O/P merB3 and O/P merR1, while MerR2 regulates O/P merR2.