Property of cold inducible DEAD-box RNA helicase in hyperthermophilic archaea

TK0306 (Tk-DeaD) of hyperthermophilic archaeon Thermococcus kodakaraensis is annotated as the DEAD-box helicase gene; nevertheless, its ortholog has not been identified in closely related genera, Pyrococcus spp., which generally grow at higher temperature than T. kodakaraensis, suggesting that the cold-inducible RNA helicase of Tk-DeaD functions under cold stress conditions. Quantitative RT-PCR revealed that Tk-deaD was more dominantly transcribed at 60 °C than at 85 °C and 93 °C in both logarithmic and stationary phases. Immunoblot analyses revealed that Tk-DeaD was detected only in logarithmic-phase cells cultivated at 60 °C but hardly detected at 85 °C and 93 °C in both phases. Tk-DeaD expression is, hence, post-transcriptionally regulated and appears under vigorous growth conditions at 60 °C. Recombinant Tk-DeaD purified to homogeneity started to unfold at 20 °C, fully unfolded at 70 °C, and exhibited maximal ATPase activity and unwinding activity specific for single-strand paired RNA at 50 °C, which is lower than the growth limit of T. kodakaraensis.