  The success of SU(5)-like gauge coupling unification boundary conditions
$g_3^2=g_2^2=5/3 g_1^2$ has biased most attempts to embed the SM interactions
into a unified structure. After discussing the limitations of the orthodox
approach, we propose an alternative that appears to be quite naturally implied
by recent developments based on D-brane physics. In this new alternative: 1)
The gauge group, above a scale of order 1 TeV, is the minimal left-right
symmetric extension $SU(3)\times SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ of the
SM; 2) Quarks, leptons and Higgs fields come in three generations; 3) Couplings
unify at an intermediate string scale $M_s= 9\times 10^{11}$ GeV with boundary
conditions $g_3^2=g_L^2=g_R^2=32/3 g_{B-L}^2$. This corresponds to the natural
embedding of gauge interactions into D-branes and is different from the
standard SO(10) embedding which corresponds to $k_{B-L}=8/3$. Unification only
works in the case of three generations;
  4) Proton stability is automatic due to the presence of $Z_2$ discrete
R-parity and lepton parities. A specific Type IIB string orientifold model with
the above characteristics is constructed. The existence of three generations is
directly related to the existence of three complex extra dimensions.
  In this model the string scale can be identified with the intermediate scale
and SUSY is broken also at that scale due to the presence of anti-branes in the
vacuum.
  We discuss a number of phenomenological issues in this model including Yukawa
couplings and a built-in axion solution to the strong-CP problem. The present
framework could be tested by future accelerators by finding the left-right
symmetric extension of the SM at a scale of order 1 TeV.
