DEAD-box proteins, a large class of RNA-dependent ATPases, regulate all aspects 
of gene expression and RNA metabolism. They can facilitate dissociation of RNA 
duplexes and remodeling of RNA-protein complexes, serve as ATP-dependent 
RNA-binding proteins, or even anneal duplexes. These proteins have highly 
conserved sequence elements that are contained within two RecA-like domains; 
consequently, their structures are nearly identical. Furthermore, crystal 
structures of DEAD-box proteins with bound RNA reveal interactions exclusively 
between the protein and the RNA backbone. Together, these findings suggest that 
DEAD-box proteins interact with their substrates in a nonspecific manner, which 
is confirmed in biochemical experiments. Nevertheless, this contrasts with the 
need to target these enzymes to specific substrates in vivo. Using the DEAD-box 
protein Rok1 and its cofactor Rrp5, which both function during maturation of the 
small ribosomal subunit, we show here that Rrp5 provides specificity to the 
otherwise nonspecific biochemical activities of the Rok1 DEAD-domain. This 
finding could reconcile the need for specific substrate binding of some DEAD-box 
proteins with their nonspecific binding surface and expands the potential roles 
of cofactors to specificity factors. Identification of helicase cofactors and 
their RNA substrates could therefore help define the undescribed roles of the 19 
DEAD-box proteins that function in ribosome assembly.