Identification and characterization of the psychrophilic bacterium CidnaK gene in the Antarctic Chlamydomoas sp. ICE-L under freezing conditions
Heat shock protein DnaK (Hsp70) can prevent irreversible protein denaturation by chaperoning the unfolded polypeptides under extremely cold environment, and enhance cold and freezing tolerance of microorganism. From an Antarctic sea-ice green alga Chlamydomonas sp. ICE-L, eight Hsp70 genes were identified and divided into seven subfamilies: four in the cytoplasmic subfamilies, two in the chloroplast subgroups, and one in the mitochondria or endoplasmic reticulum subfamilies. Phylogenetic analyses showed that except CidnaK, each of the other Cihsp70 genes had their homologs in Chlamydomonas reinhardtii and Volvox carteri. CiDnak is a cytoplasmic protein with highly homologous to DnaK proteins from prokaryotes, and it had 100% amino acid sequence identities with the DnaK of Psychroflexus torquis ATCC 700755, a psychrophilic bacterium isolated from Antarctic sea ice. The transcription of CidnaK was significantly induced upon freezing stress in ICE-L, eminently higher than that of other Cihsp70 genes. In addition, the transformed C. reinhardtii 137c with CidnaK gene showed much higher rates of survival and growth than the wild type under freezing or low temperature conditions. Our results suggested that CidnaK was likely originated from the symbiotic bacterium in the sea-ice brine, and is an important candidate freezing inducible gene that confers low temperature tolerance to Chlamydomonas cells, and plays a crucial role in ICE-L adapting the frigid Antarctic environments.
KeywordsAntarctic sea-ice Chlamydomonas Chlorophyta DnaK/Hsp70 Horizontal gene transfer Freezing tolerance
This work was financially supported by a grant of National Natural Science Foundation (41276203) and an Open Foundation of the State Key Laboratory of Bioactive Seaweed Substances (2017) for Chenlin Liu.
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