DEAD-box proteins are nonprocessive RNA helicases that play diverse roles in 
cellular processes. The Neurospora crassa DEAD-box protein CYT-19 promotes 
mitochondrial group I intron splicing and functions as a general RNA chaperone. 
CYT-19 includes a disordered, arginine-rich "C-tail" that binds RNA, positioning 
the helicase core to capture and unwind nearby RNA helices. Here we probed the 
C-tail further by varying the number and positions of arginines within it. We 
found that removing sets of as few as four of the 11 arginines reduced RNA 
unwinding activity (kcat/KM) to a degree equivalent to that seen upon removal of 
the C-tail, suggesting that a minimum or "threshold" number of arginines is 
required. In addition, a mutant with 16 arginines displayed RNA unwinding 
activity greater than that of wild-type CYT-19. The C-tail modifications 
impacted unwinding only of RNA helices within constructs that included an 
adjacent helix or structured RNA element that would allow C-tail binding, 
indicating that the helicase core remained active in the mutants. In addition, 
changes in RNA unwinding efficiency of the mutants were mirrored by changes in 
functional RNA affinity, as determined from the RNA concentration dependence of 
ATPase activity, suggesting that the C-tail functions primarily to increase RNA 
affinity. Interestingly, the salt concentration dependence of RNA unwinding 
activity is unaffected by C-tail composition, suggesting that the C-tail uses 
primarily hydrogen bonding, not electrostatic interactions, to bind 
double-stranded RNA. Our results provide insights into how an unstructured 
C-tail contributes to DEAD-box protein activity and suggest parallels with other 
families of RNA- and DNA-binding proteins.