  Recently we proposed a natural scenario of grand unified theories with
anomalous U(1)_A gauge symmetry, in which doublet-triplet splitting is realized
in SO(10) unification using Dimopoulos-Wilczek mechanism and realistic quark
and lepton mass matrices can be obtained in a simple way. The scenario has an
additional remarkable feature that the symmetry breaking scale and the mass
spectrum of super heavy particles are determined essentially by anomalous
U(1)_A charges. Therefore once all the anomalous U(1)_A charges are fixed, the
gauge coupling flows can be calculated. We examine several models in which the
gauge coupling unification is realized. Examining the conditions for the
coupling unification, we show that when all the fields except those of the
minimal SUSY standard model become super-heavy, the unification scale
generically becomes just below the usual GUT scale \Lambda_G\sim 2\times
10^{16} GeV and the cutoff scale becomes around \Lambda_G. Since the lower GUT
scale leads to shorter life time of nucleon, the proton decay via dimension six
operator p\to e^+\pi^0 can be seen in future experiment. On the other hand, the
lower cutoff scale than the Planck scale may imply the existence of extra
dimension in which only gravity modes can propagate.
