Gauging quantum-mechanical space-time symmetry

We claim that the U(I) phase invariance in Quantum Theory constitutes an essential ingredient of the (quantum)space-time symmetry. If we identify this group with a 1-dimensional subgroup of some internal rigid symmetry, then turning the space-time subgroup of the Poincare group into a local symmetry automatically promotes the original internal symmetry to the gauge level in a non-direct product way. This mechanism provides, in particular, a non-trivial mixing of the electromagnetic and gravitational forces.