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

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 (Czc-CBA1 and CzcCBA2). In this study we showed 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 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 improved Cd2+ resistance when it was expressed in Escherichia coli. CzcCBA2 contributed to Zn resistance of a czcA1-defective P. putida strain or when the CzcA2 subunit was overexpressed in a transporter-deficient strain. It seemed that CzcA2 could complex with CzcC1 and CzcB1 subunits and therefore complement the loss of CzcA1. The CzcCBA2 transporter itself, however, did not function. Expression of cadA1, cadA2, and czcCBA1 was induced by heavy metals, and the expression levels were dependent on the growth medium and growth 21 21 21 phase. Expression of cadA2 and czcCBA1 was nonspecific; both genes were induced by Zn2+, Cd2+, Pb2+, Ni2+, Co2+, and Hg2+. On the other hand, remarkably, expression of cadA1 was induced only by Zn2+. Possible roles of distinct but simultaneously functioning transporters are discussed.