  RX J1301.9+2747 is an ultrasoft active galactic nucleus (AGN) with unusual
X-ray variability that is characterized by a long quiescent state and a
short-lived flare state. The X-ray flares are found to recur quasi-periodically
on a timescale of 13-20 ks. Here, we report the analysis of the light curve in
the quiescent state from two XMM observations spanning 18.5 years, along with
the discovery of a possible quasi-periodic X-ray oscillation (QPO) with a
period of ~1500s. The QPO is detected at the same frequency in the two
independent observations, with a combined significance of >99.89%. The QPO is
in agreement with the relation between frequency and black hole mass (M_BH)
that has been reported in previous works for AGNs and Galactic black hole X-ray
binaries (XRBs). The QPO frequency is stable over almost two decades,
suggesting that it may correspond to the high-frequency type found in XRBs and
originates, perhaps, from a certain disk resonance mode. In the 3:2
twin-frequency resonance model, our best estimate on the M_BH range implies
that a maximal black hole spin can be ruled out. We find that all ultrasoft
AGNs reported so far display quasi-periodicities in the X-ray emission,
suggesting a possible link on the part of the extreme variability phenomenon to
the ultrasoft X-ray component. This indicates that ultrasoft AGNs could be the
most promising candidates in future searches for X-ray periodicities.
