  We present the results from the Suzaku observation of the powerful radio-loud
quasar RBS315 (z=2.69), for which a previous XMM-Newton observation showed an
extremely flat X-ray continuum up to 10 keV (photon index Gamma=1.26) and
indications of strong intrinsic absorption (N_H~10^22 cm^{-2} assuming neutral
gas). The instrument for hard X-rays HXD/PIN allows us a detection of the
source up to 50 keV. The broad-band continuum (0.5-50 keV) can be well modeled
with a power-law with slope Gamma=1.5 (definitively softer than the continuum
measured by XMM-Newton) above 1 keV with strong deficit of soft photons. The
low-energy cut-off can be well fitted either with intrinsic absorption (with
column density N_H~10^22 cm^{-2} in the quasar rest frame) or with a break in
the continuum, with an extremely hard (Gamma =0.7) power-law below 1 keV. We
construct the Spectral Energy Distribution of the source, using also optical-UV
measurements obtained through a quasi-simultaneous UVOT/SWIFT observation. The
shape of the SED is similar to that of other Flat Spectrum Radio Quasars
(FSRQs) with similar power, making this source an excellent candidate for the
detection in gamma-rays by GLAST. We model the SED with the synchrotron-Inverse
Compton model usually applied to FSRQs, showing that the deficit of soft
photons can be naturally interpreted as due to an intrinsic curvature of the
spectrum near the low energy end of the IC component rather than to intrinsic
absorption, although the latter possibility cannot be ruled out. We propose
that in at least a fraction of the radio-loud QSOs at high redshift the cut-off
in the soft X-ray band can be explained in a similar way. Further studies are
required to distinguish between the two alternatives.
