  We determine the persistence length, $l_p$, for a bacterial group I ribozyme
as a function of concentration of monovalent and divalent cations by fitting
the distance distribution functions $P(r)$ obtained from small angle X-ray
scattering intensity data to the asymptotic form of the calculated $P_{WLC}(r)$
for a worm-like chain (WLC). The $l_p$ values change dramatically over a narrow
range of \Mg concentration from $\sim$21 \AA in the unfolded state (\textbf{U})
to $\sim$10 \AA in the compact ($\mathrm{I_C}$) and native states. Variations
in $l_p$ with increasing \Na concentration are more gradual. In accord with the
predictions of polyelectrolyte theory we find $l_p \propto 1/ \kappa^2$ where
$\kappa$ is the inverse Debye-screening length.
