Recent insights into biological regulation from cell-free protein-synthesizing systems.

We review the important role that cell-free protein-synthesizing systems (CFPSS) have played in the history of modern biology, and highlight two recent applications that illustrate their continued utility for the exploration of otherwise intractable aspects of gene expression and its regulation. Viral capsid assembly recreated in CFPSS reveals a catalyzed biochemical pathway involving transient, energy-dependent action of host proteins and discrete assembly intermediates, rather than the classical notion of self-assembly that was expected for capsid formation. Study of prion protein biogenesis reveals a new conformation critical for disease pathogenesis and advances the paradigm of protein bioconformatics, by which cells may productively regulate the folding of various proteins. In each example, the CFPSS made it easier to analyze biochemical mechanism than is possible in other currently available whole cell systems, illustrating why this approach is likely to be a continuing source of insight into important features of biological regulation.