When proteins are made inside cells, genetic information (in the form of messenger RNA) must be "translated" into specific sequences of amino acid building blocks. Accurate translation is essential to the health of the cell, and the idea that "one gene gives one protein" emerged very early in the development of the field of molecular biology. Researchers in the Center for the Science and Engineering of Materials (CSEM) at the California Institute of Technology are changing that idea. By expressing carefully designed artificial genes in bacterial cells, CSEM investigators have created artificial proteins that show considerable promise as surgical materials and as matrices for regenerative medicine. Furthermore, by engineering the enzymes that control protein synthesis, they've expanded the set of amino acids that can be used to make proteins. They've shown, for example, that the natural amino acid phenylalanine (F) can be replaced by para-azidophenylalanine (pAzF), a photosensitive derivative. The inclusion of pAzF renders proteins susceptible to light-induced crosslinking, and because pAzF and phenylalanine compete with one another in the translation step, one can use a single gene to direct the synthesis of not just one protein, but of a whole family of related proteins. The figure shows measurements of the stress-strain behavior of one such family, in which the relative amounts of phenylalanine and pAzF were systematically varied. As the relative amount of pAzF in the protein increases, the proteins crosslink more tightly and give rise to stiffer materials. Thus while a "literal translation" of the messenger RNA would yield just one material, the introduction of pAzF enables the synthesis of many new materials on the same RNA template. The stiffness of the material (and other important properties) can therefore be matched closely to the demands of the intended application.
"Woe to the makers of literal translations, who by rendering every word weaken the meaning." Voltaire, On Tragedy (1732); quoted in The Columbia World of Quotations (1996).