  The gauge structure of the four dimensional effective theory originated in a
pure five dimensional Yang-Mills theory compactified on the orbifold $S^1/Z_2$,
is discussed on the basis of the BRST symmetry. If gauge parameters propagate
in the bulk, the excited Kaluza-Klein (KK) modes are gauge fields and the four
dimensional theory is gauge invariant only if the compactification is carried
out by using curvatures as fundamental objects. The four dimensional theory is
governed by two types of gauge transformations, one determined by the KK zero
modes of the gauge parameters and the other by the excited ones. Within this
context, a gauge-fixing procedure to quantize the KK modes that is covariant
under the first type of gauge transformations is shown and the ghost sector
induced by the gauge-fixing functions is presented. If the gauge parameters are
confined to the usual four dimensional space-time, the known result in the
literature is reproduced with some minor variants, although it is emphasized
that the excited KK modes are not gauge fields, but matter fields transforming
under the adjoint representation of $SU_4(N)$. A calculation of the one-loop
contributions of the excited KK modes of the electroweak gauge group on the
off-shell WWV, with V a photon or a Z boson, is exhibited. Such contributions
are free of ultraviolet divergences and well-behaved at high energies.
