We present a unified point of view on the different methods available
in the literature to extract gauge theory renormalization constants fr
om the low-energy limit of string theory. The Bern-Kosower method, bas
ed on an off-shell continuation of string theory amplitudes, and the c
onstruction of low-energy string theory effective actions for gauge pa
rticles, can both be understood in terms of strings interacting with b
ackground gauge fields, and thus reproduce, in the low-energy limit, t
he held theory results of the background field method. We present in p
articular a consistent off-shell continuation of the one-loop gluon am
plitudes in the open bosonic string that reproduces exactly the result
s of the background field method in the Feynman gauge.