The Neurospora crassa CYT-18 protein, the mitochondrial tyrosyl-tRNA synthetase, 
functions in splicing group I introns by inducing formation of the catalytically 
active RNA structure. Here, we identified a DEAD-box protein (CYT-19) that 
functions in concert with CYT-18 to promote group I intron splicing in vivo and 
vitro. CYT-19 does not bind specifically to group I intron RNAs and instead 
functions as an ATP-dependent RNA chaperone to destabilize nonnative RNA 
structures that constitute kinetic traps in the CYT-18-assisted RNA-folding 
pathway. Our results demonstrate that a DExH/D-box protein has a specific, 
physiologically relevant chaperone function in the folding of a natural RNA 
substrate.