US EPA

4643612 

Journal Article 

NmtA, a novel metallothionein of Anabaena sp. strain PCC 7120 imparts protection against cadmium stress but not oxidative stress 

T V, D; Chandwadkar, P; Acharya, C 

2018 

Yes 

Aquatic Toxicology
ISSN: 0166-445X
EISSN: 1879-1514 

199 

152-161 

English 

29626757 

10.1016/j.aquatox.2018.03.035 

WOS:000433271600014 

Metallothioneins (MTs) are low molecular weight, sulfhydryl-containing, cysteine-rich, metal-binding proteins. Eukaryotes have multiple metallothionein genes; however, there is dearth of reports on prokaryotic metallothioneins. Bacterial MTs with SmtA from Synechococcus PCC 7942 as prototype have been studied in the context of cadmium detoxification. In this study, a smtA related ORF, namely nmtA, was identified in the heterocystous, nitrogen-fixing cyanobacterium, Anabaena PCC 7120. A recombinant N-terminal histidine-tagged Anabaena NmtA protein was overexpressed in Escherichia coli and purified. The protein was identified by peptide mass fingerprinting using MALDI-TOF Mass Spectrometry as putative metallothionein of Anabaena PCC 7120 with a calculated mass of ∼6.1 kDa. While the native metallated NmtA exhibited resistance against proteolysis, metal free apo-NmtA resulting from acid and dithiothreitol (DTT) treatment could be digested by proteinase K revealing a metal dependent proteolytic protection of NmtA. Expression of nmtA in Anabaena PCC 7120 was induced evidently by cadmium, zinc and copper but not by uranium or hydrogen peroxide. Recombinant Anabaena PCC 7120 overexpressing NmtA protein revealed superior cadmium tolerance but showed limited influence against oxidative stress tolerance as compared with the strain carrying vector alone. In contrast, a mutant of Synechococcus PCC 7942 deficient in MT locus was found to be highly susceptible to H2O2 indicating a likely involvement of cyanobacterial MT in protection against oxidative damage. Overall, the study improved our understanding of metal tolerance mechanisms in Anabaena PCC 7120 by demonstrating a key role of NmtA in cadmium tolerance.