Proteomic analysis to understand Cr(VI) homeostasis in Streptomyces sp. MC1

Abstract

Chromium is a heavy metal widely used in a variety of industrial processes (leather tanning, steel production, metal corrosion inhibition). Hexavalent chromium is carcinogenic and presents higher toxicity than trivalent form since Cr(VI) is more water-soluble and mobile than Cr(III). Industrial effluents containing Cr(VI) are released into water courses, mostly without proper treatment, resulting in anthropogenic contamination. Over the last years, bacteria-mediated removal or stabilization of heavy metal into no or less toxic forms has become in an effective biotechnological process. In this sense, several physiological studies on Streptomyces sp. MC1, an actinobacteria isolated from a polluted soil in the province of Tucumán (Argentina), demonstrated be able to grow in presence of Cr(VI) and remove the metal both in liquid medium and contaminated soils. However, the molecular mechanisms involved are unknown in this actinobacteria. MS-based proteomics have become a powerful tool to understand the mechanisms that underlie physiological processes. In the present work, we use MS-based, label-free and quantitative proteomic analyses in order to identify enzymes involves in oxidative stress response caused by the presence of Cr(VI) in our actinobacteria strain MC1. Sampling points for proteomics analyses were established according to the growth of Streptomyces sp. MC1 in minimal medium (MM) amended with Cr(VI) at 50 mg L-1 and MM without the metal (control condition). Cells were harvested after 18 and 24 h of incubation in control condition and MM with Cr(VI) respectively. These sampling points allowed obtaining comparable and metabolically active cells (exponential phase of growth). Cr(VI) removal was 10% at the time that cells were harvested (24 h). A total of 1981 different proteins were detected in the proteome. It represents approximately 22% of the predicted protein sequences for this strain. 518 of these proteins passed our significance parameters which 186 of them were up-regulated in the condition supplemented with Cr(VI). Analysis with the software BlastKOALA showed that up-regulated proteins were distributed in metabolic pathways that result essential for a correct cellular operation. Overall, the proteins were related to carbon and energy metabolism, genetic information processing, oxidative stress response and membrane transports. Interestingly, enzymes from pentose phosphate pathway increasing significantly their abundance in presence of chromium. About, 10 different oxidoreductases enzymes were up-regulated in presence of the metal. Regarding oxidative stress response, key enzymes like superoxide dismutase, catalase, mycothiol synthase, and mycothiol amidase were identified with an increment in their abundance. The proteome analysis performed in Streptomyces sp. MC1 allowed us to identify the proteins involves in the homeostasis of Cr(VI). These results serve as basement to study and improve the heavy metal removal by actinobacteria.