Gauge coupling and fermion mass relations in low string scale brane   models
  We analyze the gauge coupling evolution in brane inspired models with $U(3) \times U(2) \times U(1)^N$ symmetry at the string scale. We restrict to the case of brane configurations with two and three abelian factors (N=2,3) and where only one Higgs doublet is coupled to down quarks and leptons and only one to the up quarks. We find that the correct hypercharge assignment of the Standard Model particles is reproduced for six viable models distinguished by different brane configurations. We investigate the third generation fermion mass relations and find that the correct low energy $m_b/m_{\tau}$ ratio can be obtained for $b-\tau$ Yukawa coupling equality at a string scale as low as $M_S \sim 10^3$ TeV. 
D.V. Gioutsos, G.K. Leontaris and J. Rizos