  We compute the amplitude of deeply virtual Compton scattering (DVCS) using
the calculus of QCD string operators in coordinate representation. To restore
the electromagnetic gauge invariance
  (transversality) of the twist-2 amplitude we include the operators of twist-3
which appear as total derivatives of twist-2 operators. Our results are
equivalent to a Wandzura-Wilczek approximation for twist-3 skewed parton
distributions. We find that this approximation gives a finite result for the
amplitude of a longitudinally polarized virtual photon, while the amplitude for
transverse polarization is divergent, i.e., factorization breaks down in this
term. However, the divergent part has zero projection onto the polarization
vector of the final real photon.
