  Generic D-brane string models of particle physics predict the existence of
extra U(1) gauge symmetries beyond hypercharge. These symmetries are not of the
E_6 class but rather include the gauging of Baryon and Lepton numbers as well
as certain Peccei-Quinn-like symmetries. Some of the U(1)'s have triangle
anomalies, but they are cancelled by a Green-Schwarz mechanism. The
corresponding gauge bosons typically acquire a mass of order the string scale
M_S by combining with two-index antisymmetric fields coming from the closed
string sector of the theory. We argue that in string models with a low string
scale M_S proportional to 1-10 TeV, the presence of these generic U(1)'s may be
amenable to experimental test. Present constraints from electroweak precision
data already set important bounds on the mass of these extra gauge bosons. In
particular, for large classes of models, rho-parameter constraints imply M_S >=
1.5 TeV. In the present scheme some fraction of the experimentally measured Z^0
mass would be due not to the Higgs mechanism, but rather to the mixing with
these closed string fields. We give explicit formulae for recently constructed
classes of intersecting D6- and D5-brane models yielding the Standard Model
(SM) fermion spectrum.
