Ribonuclease P (RNase P) catalyzes the 5′
maturation of precursor tRNA transcripts and, in bacteria,
is composed of a catalytic RNA and a protein. We investigated
the oligomerization state and the shape of the RNA alone
and the holoenzyme of Bacillus subtilis RNase
P in the absence of substrate by synchrotron small-angle
X-ray scattering and affinity retention. The B. subtilis
RNase P RNA alone is a monomer; however, the scattering
profile changes upon the addition of monovalent ions, possibly
suggesting different interdomain angles. To our surprise,
the X-ray scattering data combined with the affinity retention
results indicate that the holoenzyme contains two RNase
P RNA and two RNase P protein molecules. We propose a structural
model of the holoenzyme with a symmetrical arrangement
of the two RNA subunits, consistent with the X-ray scattering
results. This (P RNA)2(P protein)2
complex likely binds substrate differently than the conventional
(P RNA)1(P protein)1 complex; therefore,
the function of the B. subtilis RNase P holoenzyme
may be more diverse than previously thought. These revisions
to our knowledge of the RNase P holoenzyme suggest a more
versatile role for proteins in ribonucleoprotein complexes.