Chemotactic transduction in biomining microorganisms

The oxidative attack of ores by bioleaching microorganisms requires that the cells adhere to specific sites on the surface of the minerals. This attachment will depend on the sensing by the microorganisms of a dissolved ion concentration gradient present in the immediate vicinity of the solid. Chemotactic responses towards metal ions and other compounds have been described for Thiobacillus ferrooxidans (now Acidithiobacillus ferrooxidans) and Leptospirillum ferrooxidans. More recently, we isolated and characterized a gene from L. ferrooxidans coding for a putative chemotactic receptor (LcrI). I review here what is known about the chemotactic behaviour of biomining microorganisms, and analyze further the characteristics of LcrI. The LcrI-deduced protein contained two hydrophobic regions most likely corresponding to the two transmembrane regions present in all the methyl-accepting chemotaxis proteins (MCPs) which produce a folded protein with both a periplasmic and a cytoplasmic domain. We have proposed that the cytoplasmic domain of LcrI also contains a highly conserved domain (HCD region) present in all bacterial chemotactic receptors, and two probable methylation sites. The periplasmic domain of LcrI showed a very high positive charge at acidic pH when compared with periplasmic domains of a non-acidophilic bacterium. This charge difference may represent a special adaptation of acidophilic microorganisms such as T. ferrooxidans and L. ferrooxidans that have to sense effectors at the very low pH of their periplasms. On the other hand, the presence in LcrI of two putative amphipathic helices, characteristic of the cytoplasmic domains of MCPs, that may have a role in transmembrane signal transduction, suggests that the signaling mechanism mediated by the cytoplasmic region of LcrI would be essentially the same in the acidophilic chemolithoautotrophic biomining microorganisms when compared with other microorganisms.