  We consider a quark-lepton symmetry model of unification of the strong and
electromagnetic interactions. The model has the gauge group $SU(4)\times
U(1)_{Y}$ and the minimal Higgs structure consisting of one complex quartet of
scalar particles. The spontaneous breakdown of the gauge group to
$SU(3)_{c}\times U(1)_{em}$ due to nonzero vacuum expectation value of the
Higgs quartet provides the simplest realization of the Higgs mechanism which
generates masses for gauge bosons, and masses to quarks and leptons. Using the
embedding of the gauge group to SU(5), we study the evolution of coupling
constants and find a connection of the couplings with the gauge couplings of
the standard model.
