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J. Bacteriol. doi:10.1128/JB.01494-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The interplay of different transporters in the mediation of divalent heavy metal resistance in Pseudomonas putida KT2440

Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki, Finland; Laboratory of Molecular Genetics, National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

^* To whom correspondence should be addressed. Email: anu.leedjarv{at}helsinki.fi.

	Abstract

According to in silico analysis, the genome of Pseudomonas putida KT2440 encodes at least four Zn/Cd/Pb efflux transporters – two P-type ATPases, CadA1 and CadA2, and two czc chemiosmotic transporters, CzcCBA1 and CzcCBA2. In this study we show that all these transporters are functional, but under laboratory conditions only two of them were involved in the mediation of heavy metal resistance in P. putida KT2440. CadA2 conferred to Cd²⁺ and Pb²⁺ resistance, whereas CzcCBA1 was involved in export of Zn²⁺, Cd²⁺, and possibly Pb²⁺. CadA1, although nonfunctional in P. putida, improved Zn²⁺ resistance and slightly Cd²⁺ resistance, when expressed in Escherichia coli. CzcCBA2 contributed to Zn resistance of a czcA1-defective P. putida strain, or when the CzcA2 subunit was overexpressed in transporter-deficient strain. It rather seemed that CzcA2 could complex with CzcC1 and CzcB1 subunits, and therefore complement the loss of CzcA1. The CzcCBA2 transporter itself was not, however, functioning. Expression of cadA1, cadA2, and czcCBA1 was induced by heavy metals, and expression levels were dependent on growth medium and growth phase. Expression of cadA2 and czcCBA1 was non-specific, both were induced by Zn²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Co²⁺, and Hg²⁺. On the other hand, expression of cadA1 was remarkably induced only by Zn²⁺. Possible roles of distinct, but simultaneously functioning transporters are discussed.