Pseudomonas sp. B-33 isolated from the contaminated environment is capable of growth in the presence of 100uM HgCl

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. Three kinds of plasmids were detected in agarose gel electrophoresis. The mercuric reducing activity was measured and verified in the strain by atomic absorption spectroscopy. A cured derivative of B-33 was obtained by NTG treatment. This strain lacks one of the 3 plasmids in B-33 and fails to grow in the pressence of 100uM HgCl

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. The mercuric reducing ability was markedly decreased in the cured strain. The cured plasmid （designated pBH33） was isolated and purified by electrophoresis and open circular DNA molecules were observed under electron microscpe. There are one site for EcoRI

two sites for PstI and four sites for Hind Ⅲ The molecular weight of pBH33 was estimated to be about 7.24×10

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d. by agarose electrophoresis. The cured strain was transformed by pBH33 and mercuric resistant ability was recovered.