DEAD-box RNA helicase involved in the assembly of the 50S ribosomal subunit. Has an RNA-dependent ATPase activity, which is specific for 23S rRNA, and a 3' to 5' RNA helicase activity that uses the energy of ATP hydrolysis to destabilize and unwind short rRNA duplexes (Probable). ATP + H2O = ADP + H(+) + phosphate ATPase activity is stimulated by interaction with RNA. May interact with RNA helicases CshA and CshB. In rich medium highest expression in exponential growth, expression decreases in stationary phase (at protein level). Contains an N-terminal domain that binds non-specifically to RNA and a C-terminal domain that binds specifically and tightly to hairpin 92 of 23S rRNA. Undergoes a conformation change in the helicase core upon binding of RNA and ATP. No visible effect at 37 or 16 degrees Celsius; no change in ribosome profiles. A quadruple disruption of all RNA helicases (cshA, cshB, deaD, yfmL) is not lethal at 37 degrees Celsius, although both 50S and 70S ribosomes are decreased, while growth stops at 16 degrees. Belongs to the DEAD box helicase family. DbpA subfamily.