  As a consequence of the rugged landscape of RNA molecules their folding is
described by the kinetic partitioning mechanism according to which only a small
fraction ($\phi_F$) reaches the folded state while the remaining fraction of
molecules is kinetically trapped in misfolded intermediates. The transition
from the misfolded states to the native state can far exceed biologically
relevant time. Thus, RNA folding in vivo is often aided by protein cofactors,
called RNA chaperones, that can rescue RNAs from a multitude of misfolded
structures. We consider two models, based on chemical kinetics and chemical
master equation, for describing assisted folding. In the passive model,
applicable for class I substrates, transient interactions of misfolded
structures with RNA chaperones alone are sufficient to destabilize the
misfolded structures, thus entropically lowering the barrier to folding. For
this mechanism to be efficient the intermediate ribonucleoprotein (RNP) complex
between collapsed RNA and protein cofactor should have optimal stability. We
also introduce an active model (suitable for stringent substrates with small
$\phi_F$), which accounts for the recent experimental findings on the action of
CYT-19 on the group I intron ribozyme, showing that RNA chaperones does not
discriminate between the misfolded and the native states. In the active model,
the RNA chaperone system utilizes chemical energy of ATP hydrolysis to
repeatedly bind and release misfolded and folded RNAs, resulting in substantial
increase of yield of the native state. The theory outlined here shows, in
accord with experiments, that in the steady state the native state does not
form with unit probability.
