  A geometric construction for obtaining a prolongation of a connection to a
connection of a bundle of connections is presented. This determines a natural
extension of the notion of canonical energy-tensor which suits gauge and
gravitational fields, and shares the main properties of the energy-tensor of a
matter field in the jet space formulation of Lagrangian field theory, in
particular with regards to symmetries of the Poincar\'e-Cartan form.
Accordingly, the joint energy-tensor for interacting matter and gauge fields
turns out to be a natural geometric object, whose definition needs no auxuliary
structures. Various topics related to energy-tensors, symmetries and the
Einstein equations in a theory with interacting matter, gauge and gravitational
fields can be viewed under a clarifying light. Finally, the symmetry determined
by the "Komar superpotential" is expressed as a symmetry of the gravitational
Poincar\'e-Cartan form.
