  We present a detailed dynamical model of the behavior of
transcription-translation circuits in vitro that makes explicit the roles
played by essential molecular resources. A set of simple two-gene test circuits
operating in a cell-free biochemical 'breadboard' validate this model and
highlight the consequences of limited resource availability. In particular, we
are able to confirm the existence of biomolecular 'crosstalk' and isolate its
individual sources. The implications of crosstalk for biomolecular circuit
design and function are discussed.
