  The minimal supersymmetric standard model (MSSM), is perhaps the leading
candidate for new physics beyond the standard model, but it encounters
difficulties with string gauge unification and in addition does not shed any
light on the question of fermion masses. We consider a scenario in which the
MSSM is valid up to an energy scale of 10^16 GeV, but that above this scale the
theory is supplemented by extra vector-like representations of the gauge group,
plus a gauged U(1)_X family symmetry. In our approach the extra heavy matter
above the scale 10^16 GeV is used in two different ways: (1) to allow
(two-loop) gauge coupling unification at the string scale; (2) to mix with
quarks, leptons and Higgs fields via spaghetti diagrams and so lead to
phenomenologically acceptable Yukawa textures. We determine the most economical
models in which the extra matter can satisfy both constraints simultaneously.
We then give a general discussion of the infra-red fixed points of such models,
pointing out the conditions for infra-red stability, then discuss two
semi-realistic examples: a Higgs mixing model, and a quark mixing model.
