Thioredoxin is involved in hexavalent chromium reduction in Streptomyces violaceoruber strain LZ-26-1 isolated from the Lanzhou reaches of the Yellow River

•We have isolated an actinomycete strain Streptomyces violaceoruber LZ-26-1 that can reduce Cr(VI) to Cr(III).•We found that the Cr(VI) reduction of LZ-26-1 was enhanced by NADPH and inhibited by Cd2+.•Transmission electron microscopy images showed that chromium was precipitated inside the cells.•We found a thioredoxin operon in the genome of this strain is upregulated in response to Cr(VI).

Hexavalent chromium [Cr(VI)] is one of the major environmental contaminants in the Lanzhou reaches of the Yellow River, and actinomycetes' Cr(VI) bioremediation within this area is still scarce. In the present study, twenty actinomycete strains were isolated from the Lanzhou reaches which could resist 1.2 mM Cr(VI) on Starch-Casein (SC) agar plates. Based on the 16S rRNA sequence data, sixteen strains belong to Streptomyces and four other strains belong to Gordonia, Nocardiopsis, Nocardia and Cellulosimicrobium. One strain named LZ-26-1 which is closely related to Streptomyces violaceoruber demonstrated a notable ability of Cr(VI) reduction. It could reduce 92.86% of the 0.6 mM Cr(VI) to trivalent chromium [Cr(III)] in SC liquid medium in 144 h. The optimum temperature and pH were 28 °C and 7.0 for Cr(VI) reduction. Transmission electron microscopy images showed that chromium was precipitated inside the cells. In resting cells and crude chromate reductase assays, strain LZ-26-1's Cr(VI) reduction was stimulated when NADPH was used as an electron donor and Cd2+ treatment inhibited Cr(VI) reduction. Furthermore, thioredoxin operon, which consists of three genes encoding a thioredoxin, thioredoxin reductase (NADPH) and a possible membrane protein, was upregulated by Cr(VI) treatment. These results suggested that LZ-26-1 might utilize a thioredoxin pathway to reduce Cr(VI).