  We calculate the total cross section for deep inelastic scattering (DIS) on a
nucleus at high energy for a strongly coupled N=4 super Yang-Mills theory using
AdS/CFT correspondence. In analogy to the small coupling case we argue that at
high energy the total DIS cross section is related to the expectation value of
the Wilson loop formed by the quark-antiquark dipole. We model the nucleus by a
metric of a shock wave in AdS_5. We then calculate the expectation value of the
Wilson loop by finding the extrema of the Nambu-Goto action for an open string
attached to the quark and antiquark lines of the loop in the background of an
AdS_5 shock wave. We find three extrema of the Nambu-Goto action: the string
coordinates at the extrema are complex-valued and are given by three different
branches of the solution of a cubic equation. The physically meaningful
solutions for the total DIS cross section are given either by the only branch
with a purely imaginary string coordinate in the bulk or by a superposition of
the two other branches. For both solutions we obtain the forward scattering
amplitude N for the quark dipole-nucleus scattering. We study the onset of
unitarity with increasing center-of-mass energy and transverse size of the
dipole: we observe that for both solutions the saturation scale, while
energy-dependent at lower energies, at very high energy becomes independent of
energy/Bjorken-x. The saturation scale depends very strongly on the atomic
number of the nucleus as Q_s ~ A^{1/3}.
