  We report the detection of the CO J=1-0 emission line in [H89]1821+643, one
of the most optically luminous QSOs in the local Universe and a template
ULIRG-to-QSO transition object, located in a rich cool-core cluster at z=0.297.
The CO emission is likely to be extended, highly asymmetric with respect to the
center of the host elliptical where the QSO resides, and corresponds to a
molecular gas mass of ~ 8.0 x 10^{9} M_sun. The dynamical mass enclosed by the
CO line emission region could amount up to ~ 1.7x10^{12} M_sun (80% of the
total mass of the elliptical host). The bulk of the CO emission is located at
~9 kpc south-east from the nuclei position, and close to a faint optical
structure, suggesting that the CO emission could either represent a gas-rich
companion galaxy merging with the elliptical host or a tail-like structure
reminiscent of a previous interaction. We argue that the first scenario is more
likely given the large masses implied for the CO source, which would imply a
highly asymmetrical elliptical host. The close alignment between the CO
emission major axis and the radio-plume suggests a possible role of the latter
excitation of the ambient gas reservoir. The stacking technique was used to
search for CO emission and 3mm continuum emission from galaxies in the
surrounding cluster, however no detection was found either toward individual
galaxies or the stacked ensemble of galaxies, with a 3 sigma limit of <
1.1x10^9 M_sun for the molecular gas.
