  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 lower
bound on the vector leptoquark mass originated from the low-energy rare
processes, strongly suppressed in the standard model.
