The broad-spectrum mercurial-resistance plasmid pDU1358 was analyzed by cloning the resistance determinants and preparing a physical and genetic map of a 45-kilobase (kb) region of the plasmid that contains two separate mercurial-resistance operons that mapped about 20 kb apart. One encoded narrow-spectrum mercurial resistance to Hg2+ and a few organomercurials; the other specified broad-spectrum resistance to phenylmercury and additional organomercurials. Each determinant governed mercurial transport functions. Southern DNADNA hybridization experiments using gene-specific probes from the plasmid R100 mer operon indicated close homology with the R100 determinant. The 2153 base pairs of the promoter-distal part of the broadspectrum Hg2+-resistance operon of pDU1358 were sequenced. This region included the 3'-terminal part of the merA gene, merD, unidentified reading frame URF1, and a part of URF2 homologous to previously sequenced determinants of plasmid R100. Between the merA and merD genes, an open reading frame encoding a 212 amino acid polypeptide was identified as the merB gene that determines the enzyme organomercurial lyase that cleaves the C-Hg bond of phenylmercury.Resistance to mercurial compounds is widespread in prokaryotes and is usually specified by genes on plasmids or transposons (1-6). Two distinct mercurial-resistance phenotypes have been described: (i) resistance to mercuric ions (Hg2+) by enzymatic reduction to Hg°and to a limited number of organomercurials by an unknown mechanism (narrow spectrum) and (ii) resistance in addition to a wider range of organomercurials including phenylmercuric acetate (PhHgOAc) and thimerosal (broad spectrum). Both narrowand broad-spectrum determinants specify the enzyme mercuric reductase that converts Hg2+ to the less toxic Hg°, which is then volatilized. Broad-spectrum determinants encode, in addition, the enzyme organomercurial lyase, which cleaves C-Hg bonds to yield Hg2+ ions, which are then detoxified by mercuric reductase.The mercury-resistance determinants also encode transport functions, which carry mercurial compounds across the cell membrane and present the mercurials to the detoxifying enzymes. If the transport functions are expressed in the absence of reductase activity, the cells become hypersensitive to mercurial compounds (7).The narrow-spectrum mer resistance determinants from plasmid R100 (which contains transposon Tn2l encoding mercury resistance) and transposon TnSOI have been studied by genetic and DNA sequence analysis (8-13). The merT and merP gene products are involved in the uptake and transport of Hg2+; merA specifies the mercuric reductase enzyme; merC, merD, and the unidentified reading frames (URF1 and URF2) are of uncertain function. The merR gene of R100 is transcribed divergently from merT, merP, merC, merA, and merD and specifies a regulatory protein that normally represses the transcription of the mer operon (14). However, in the presence of subtoxic concentrations of Hg2+, the merR product acts as an inducer of the merc...

SummaryGlobal regulatory circuits of the type mediated by CRP and FNR in Escherichia coli were sought in Lactococcus lactis to provide a basis for redirecting carbon metabolism to speci®c fermentation products. Using a polymerase chain reaction (PCR) approach, two genes (¯pA and¯pB ) encoding FNR-like proteins (FlpA and FlpB) with the potential for mediating a dithiol-disulphide-dependent regulatory switch, were identi®ed. Transcript analysis indicated that they are distal genes of two paralogous operons, orfX-orfYp, in which the orfX and orfY genes were predicted to encode binding domain components of cation ATPases and storage proteins respectively. The corresponding promoters were each associated with a potential FNR site (TTGAT----ATCAA) at positions 4.5 (¯pA operon) and À42.5 (¯pB operon), suggesting that the respective operons might be negatively and positively autoregulated. The incomplete open reading frames (orfW A /B ) located upstream of each operon were predicted to encode additional components of paralogous cation ATPases. No phenotypic effects were detected in¯pA and¯pB single mutants, but the double mutant had a lower intracellular zinc content, an increased sensitivity to hydrogen peroxide and an altered polypeptide pro®le (as determined by two-dimensional gel electrophoresis): formate production was not affected. It was concluded tentatively that FlpA and FlpB regulate overlapping modulons, including systems concerned with zinc uptake, in response to metal ion or oxidative stress.

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