Chkareuli and I.G

The significant heavy threshold effect is found in the supersymmetric SU(5) model with two adjoint scalars, one of which is interpreted as a massive string mode decoupled from the lower-energy particle spectra. This threshold related with the generic mass splitting of the basic adjoint moduli is shown to alter properly the running of gauge couplings, thus giving a natural solution to the string-scale grand unification as prescribed at low energies by LEP precision measurements and minimal particle content. The further symmetry condition of the (top-bottom) Yukawa and gauge coupling superunification at a string scale results in the perfectly working predictions for the top and bottom quark masses in the absence of any large supersymmetric threshold corrections.