Splicing on-off switches into proteins.

Splicing on-off swithches into proteins

The virus T4 breaks into bacterial cell walls and replicates once inside. Using a cutting enzyme called a lysozyme, the virus snips throught the wall -- a fence of interconnected sugar and protein molecules. To work, the lysozyme's "mouth," or active site, must have access to the wall's sugar units. Scientists interested in engineering enzymes for new uses report controlling the enzyme's cutting activity by building a reversible barrier across its mouth.

Using a genetic engineering technique called oligonucleotide directed mutagenesis, Brian W. Matthews and Masazumi Matsumura of the University of Oregon in Eugene replaced each of two amino acids near the enzyme's active site with cysteine, a sulfur-containing amino acid. Since the substitute cysteines were close to each other in the mutant enzyme's structure, their sulfur atoms could join into a disulfide bond. "Our results demonstrate the novel use of an engineered disulfide bond for the control of enzyme function," the researchers write in the Feb. 10 SCIENCE. The mutant enzyme molecules cannot perrform their usual job because the disulfide bond blocks their active sites, the researchers found. But by breaking the bond with reducing agents such as dithiothreitol, the scientists can reopen the active site and restore most of the enzyme's bactericidal activity.