The third type of fermion mixing and indirect limits on the Pati-Salam leptoquark mass

The low-energy manifestations of a minimal extension of the electroweak standard model based on the quark-lepton symmetry $SU(4)_V otimes SU(2)_L otimes G_R$ of the Pati-Salam type are analyzed. Given this symmetry the third type of mixing in the interactions of the $SU(4)_V$ leptoquarks with quarks and leptons is shown to be required. An additional arbitrariness of the mixing parameters could allow, in principle, to decrease noticeably the indirect lower bound on the vector leptoquark mass originated from the low-energy rare processes, strongly suppressed in the standard model.