ATP-dependent RNA helicase involved nonsense-mediated mRNA decay and ribosome biogenesis through rRNA processing. ATP + H2O = ADP + H(+) + phosphate Ubiquitous. Preferentially expressed in flowers and roots. Down-regulated by abscisic acid (ABA), salt and cold treatments. The Q motif is unique to and characteristic of the DEAD box family of RNA helicases and controls ATP binding and hydrolysis. May be due to a competing acceptor splice site. Belongs to the DEAD box helicase family. DDX5/DBP2 subfamily.