  We consider an arbitrary supersymmetric U(1) gauge factor at the TeV scale,
under which the two Higgs superfields H_{1,2} of the standard model are
nontrivial. We assume that there is a singlet superfield S such that H_1 H_2 S
is an allowed term in the superpotential. We discuss first the generic
consequences of this hypothesis on the structure of the two-doublet Higgs
sector at the electroweak energy scale, as well as Z-Z' mixing and the
neutralino sector. We then assume the existence of a grand unified symmetry and
universal soft supersymmetry breaking terms at that scale. We further assume
that the additional U(1) is broken radiatively via a superpotential term of the
form h h^c S, where h and h^c are exotic color-triplet fields which appear in
E_6 models. We show that the U(1) breaking scale and the parameter tan(beta)
\equiv v_2/v_1 are then both predicted as functions of the H_1 H_2 S coupling.
