Mercury Resistance in Bacillus cereus RC607: Transcriptional Organization and Two New Open Reading Frames

The chromosomal mercury resistance determinant of Bacillus cereus RC607 confers resistance to inorganic mercury and to organomercurials. The order of genes in the completed mercury resistance determinant is operator-promoter 1 (O/P1) merR1 merT open reading frame 3 (ORF3) ORF4 merA O/P2 merR2 merB2 merB1. The previously undetermined 1-kb DNA sequence between the merA and merB1 genes includes two significant ORFs, whose predicted protein products are homologous with MerR (the transcriptional regulator) and MerB (the organomercurial lyase enzyme). Two transcriptional start sites (promoters), O/P1 at the beginning of the determinant and O/P2 immediately upstream of the sixth ORF, the newly identified merR2, were mapped by reverse transcriptase (RT) primer extension. A long 6.3-kb mRNA traversing all eight ORFs was shown by RT-PCR. Growth sensitivity measurements in liquid media and cellular mercury volatization assays characterized inducibility and differences in functional activity in B. cereus RC607 and after cloning of the mer determinant into plasmids in Escherichia coli.