  We study in detail the GR models of the X-ray spectral variability for
various geometries of the X-ray source and with various relativistic effects
being the dominant cause of spectral variability. The predicted properties are
compared with the Suzaku observational data of the Seyfert 1 galaxy
MCG--6-30-15. The data disfavor models with the X-ray source (1) moving
vertically on the symmetry axis or (2) corotating with the disc and changing
height not far above the disc surface. The most likely explanation for the
observed variability is given by the model involving the X-ray source located
at a very small, varying distance from a rapidly rotating black hole. This
model predicts some enhanced variations in the red wing of the Fe line, which
are not seen in the Suzaku observations. However, the enhanced variability of
the red wing, while ruled out by the Suzaku data, is consistent with an excess
RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM
observations. We speculate that the presence or lack of such a feature is
related to the change of the ionization state of the innermost part of the
disc, however, investigation of such effects is currently not possible in our
model (where a neutral disc is assumed). If the model, completed by description
of ionization effects, proves to be fully consistent with the observational
data, it will provide a strong indication that the central black hole in
MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the
Fe line profile.
