DEAD-box RNA helicase involved in the assembly of the 50S ribosomal subunit at low temperature. Exhibits RNA-stimulated ATP hydrolysis and RNA unwinding activity. Acts before DeaD. ATP + H2O = ADP + H(+) + phosphate Interacts with the 50S ribosomal subunit. Forms a complex with the 50S ribosomal proteins L4 and L24, and a region near the 5'-end of 23S rRNA. The C-terminal extension is not essential for ribosome assembly, but is important for the formation or the stability of the complex. Cells lacking this gene grow very poorly at 20 degrees Celsius, show a severe deficit of free 50S ribosomal subunits and accumulate an abnormal large ribosomal subunit. Disruption also impairs the processing of both 23S and 16S rRNAs. Overexpression suppresses a mutant defective in 50S ribosomal subunit assembly. Belongs to the DEAD box helicase family. SrmB subfamily.