  Backward photoproduction of $\omega$ off a proton target is investigated in a
Reggeized model where the $u$-channel nucleon Reggeon is constructed from the
nucleon Born terms in a gauge invariant way. The $t$-channel meson exchanges
are considered as a background. While the $N_\alpha$ trajectory of the nucleon
Reggeon reproduces the overall shape of NINA data measured at the Daresbury
Laboratory in the range of $u$-channel momentum transfer squared $-1.7<u<0.02$
GeV$^2$ and energies at $E_\gamma=2.8$, 3.5 and 4.7 GeV, a possibility of
parton contributions is searched for through the nucleon isoscalar form factor
which is parameterized in terms of parton distributions at the $\omega NN$
vertex. Detailed analysis is presented for NINA data to understand the reaction
mechanism that could fill up the deep dip from the nucleon Reggeon at momentum
squared $u=-0.15$ GeV$^2$. The angle dependence of differential cross sections
at the NINA energies above are reproduced in the overall range of $u$ including
the CLAS data at forward angles in addition. The energy dependence of the
differential cross section is investigated based on the NINA data and the
recent LEPS data. A feature of the present approach that implicates parton
contributions via nucleon form factor is illustrated in the total and
differential cross sections provided by GRAAL and CB-ELSA Collaborations.
