Extracytoplasmic storage as the nickel resistance mechanism in a natural isolate ofPseudomonas putidaS4

Abstract

Metal resistances in microbes are important to study not only to understand metal homeostasis but also to use such organisms further in environmental bioremediation. Nickel (Ni²⁺) is an important micronutrient, which at higher concentration becomes toxic. Many Ni²⁺-resistant organisms are known, which resist metal by active efflux. Pseudomonas putida S4, a natural isolate from India, is reported to show a multi-metal resistance profile. In the present study, the Ni²⁺-resistance mechanism in strain S4 was examined. Wild-type cells gradually accumulated Ni²⁺but kept it preferentially in the periplasmic space in a bound form. In Ni²⁺-sensitive mutants, periplasmic storage was disturbed and more metal accumulated cytoplasmically, producing toxicity. Sodium dodecyl sulphate – polyacrylamide gel electrophoresis analysis of periplasmic proteins revealed a band of approximately 18 kDa, which appeared only in Ni²⁺-exposed wild-type cells, and which was absent from cells not exposed to Ni²⁺as well as from Ni²⁺-sensitive mutants. On the basis of these observations, we propose a Ni²⁺-resistance mechanism in P. putida S4 based on sequestration of metal in the periplasmic space. This is the first study of sequestration-based Ni²⁺resistance.Key words: nickel, resistance, periplasmic storage, Pseudomonas putida.