  Many signalling functions in molecular biology require proteins bind to
substrates such as DNA in response to environmental signals such as the
simultaneous binding to a small molecule. Examples are repressor proteins which
may transmit information via a conformational change in response to the ligand
binding. An alternative entropic mechanism of ``allostery'' suggests that the
inducer ligand changes the intramolecular vibrational entropy not just the
static structure. We present a quantitative, coarse-grained model of entropic
allostery that suggests design rules for internal cohesive potentials in
proteins employing this effect. It also addresses the issue of how the signal
information to bind or unbind is transmitted through the protein. The model may
be applicable to a wide range of repressors and also to signalling in
transmembrane proteins.

Biomolecules