  We measure the nuclear quadrupole resonance (NQR) signal on the Zn site in
nearly optimally doped YBa$_2$Cu$_3$O$_{6.92}$, when Cu is substituted by 3\%
of isotopically pure $^{67}$Zn. We observe that Zn creates large insulating
islands, confirming two earlier conjectures: that doping provokes an orbital
transition in the CuO$_2$ plane, which is locally reversed by Zn substitution,
and that the islands are antiferromagnetic. Also, we find that the Zn impurity
locally induces a breaking of the D$_4$ symmetry. Cluster and DFT calculations
show that the D$_4$ symmetry breaking is due to the same partial lifting of
degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in
La$_{2-x}$Ba$_x$CuO$_4$, similarly well-known to strongly suppress
superconductivity. These results show that in-plane oxygen $2p^5$ orbital
configurations are principally involved in the metallicity and
superconductivity of all high-T$_c$ cuprates, and provide a qualitative
symmetry-based constraint on the SC mechanism.
