Abstract
DEAD-box RNA helicases, by unwinding duplex RNA in bacteria and eukaryotes, are involved in essential cellular processes, including translation initiation and ribosome biogenesis, and have recently been implicated in enabling bacteria to survive cold-shock and grow at low temperature. Despite these critical physiological roles, they have not been characterized in archaea. Due to their presumed importance in removing cold-stabilised secondary structures in mRNA, we have characterised a putative DEAD-box RNA helicase gene (deaD) from the Antarctic methanogen, Methanococcoides burtonii. The encoded protein, DeaD is predicted to contain a core element involved in ATP hydrolysis and RNA-binding, and an unusual C-terminal domain that contains seven perfect, trideca-peptide, direct repeats that may be involved in RNA binding. Alignment and phylogenetic analyses were performed on the core regions of the M. burtonii and other DEAD-box RNA helicases. These revealed a loose but consistent clustering of archaeal and bacterial sequences and enabled the generation of a prokaryotic-specific consensus sequence. The consensus highlights the importance of residues other than the eight motifs that are often associated with DEAD-box RNA helicases, as well as de-emphasising the importance of the "A" residue within the "DEAD" motif. Cells growing at 4oC contained abundant levels of deaD mRNA, however no mRNA was detected in cells growing at 23oC (the optimal temperature for growth). The transcription initiation site was mapped downstream from an archaeal box-A element (TATA box), which preceded a long (113 nucleotides) 5'-untranslated region (5'-UTR). Within the 5'-UTR was an 11 bp sequence that closely matches (nine out of 11) cold-box elements that are present in the 5'-UTRs of cold-shock induced genes from bacteria. To determine if the archaeal 5'-UTR performs an analagous function to the bacterial 5'-UTRs, the archaeal deaD 5'-UTR was transcribed in E. coli under the control of the cspA promoter and transcriptional terminator. It has previously been reported that overexpression of the cspA 5'-UTR leads to an extended cold-shock response due to the 5'-UTR titrating cellular levels of a cold-shock repressor protein(s). In our hands, the cold-shock protein profiles resulting from overexpression of Escherichia coli cspA and M. burtonii deaD 5'-UTRs were similar, however they did not differ from those for the overexpression of a control plasmid lacking a 5'-UTR. In association with other recent data from E. coli, our results indicate that the role of the 5'-UTR in gene regulation is presently unclear. Irrespective of the mechanisms, it is striking that highly similar 5'-UTRs with cold-box elements are present in cold induced genes from E. coli, Anabaena and M. burtonii. This is the first study examining low temperature regulation in archaea and provides initial evidence that gene expression from a cold adapted archaeon involves a bacterial-like transcriptional regulatory mechanism. In addition, it provides the foundation for further studies into the function and regulation of DEAD-box RNA helicases in archaea, and in particular, their roles in low temperature adaptation.