  It is shown that the effective five-dimensional theory of the strongly
coupled heterotic string is a gauged version of N=1 five-dimensional
supergravity with four-dimensional boundaries. For the universal
supermultiplets, this theory is explicitly constructed by a generalized
dimensional reduction procedure on a Calabi-Yau manifold. A crucial ingredient
in the reduction is the retention of a "non-zero mode" of the four-form field
strength, leading to the gauging of the universal hypermultiplet by the
graviphoton. We show that this theory has an exact three-brane domain wall
solution which reduces to Witten's "deformed" Calabi-Yau background upon
linearization. This solution consists of two parallel three-branes with sources
provided by the four-dimensional boundary theories and constitutes the
appropriate background for a reduction to four dimensions. Four-dimensional
space-time is then identified with the three-brane worldvolume.
