Heavy metal tolerance and metal homeostasis in Pseudomonas putida as revealed by complete genome analysis

The genome of Pseudomonas putida KT2440 encodes an unexpected capacity to tolerate heavy metals and metalloids. The availability of the complete chromosomal sequence allowed the categorization of 61 open reading frames likely to be involved in metal tolerance or homeostasis, plus seven more possibly involved in metal resistance mechanisms. Some systems appeared to be duplicated. These might perform redundant functions or be involved in tolerance to different metals. In total, P. putida was found to bear two systems for arsenic (arsRBCH), one for chromate (chrA), four to six systems for divalent cations (two cadA and two to four czc chemiosmotic antiporters), two systems for monovalent cations: pacS, cusCBA (plus one cryptic silP gene containing a frameshift mutation), two operons for Cu chelation (copAB), one metallothionein for metal(loid) binding, one system for Te/Se methylation (tpmT) and four ABC transporters for the uptake of essential Zn, Mn, Mo and Ni (one nikABCDE, two znuACB and one mobABC). Some of the metal-related clusters are located in gene islands with atypical genome signatures. The predicted capacity of P. putida to endure exposure to heavy metals is discussed from an evolutionary perspective.