Traditional genetic engineering involves sticking a foreign gene into bacteria and using the bacteria as tiny factories to make the protein encoded by that gene. This approach wouldn’t work for all silica-forming proteins found in marine sponges. The minerals produced by these proteins, which the researchers want to study, can kill the cells.
So Daniel Morse, of the University of California, Santa Barbara, and his colleagues looked to another protein making strategy: synthetic cells with a tiny plastic bead nucleus surrounded by a bubble of oil that acts as a cell membrane.
The scientists attached a piece of DNA to each of the beads, encoding a unique silica-forming protein, or silicatein. This DNA is a random combination of genes from two related silicateins, interspersed with random mutations.
Then the scientists soaked the beads in watery mixture of the bacterial proteins necessary to turn the DNA into silicateins and covered each bead with a thin layer of oil, trapping water and the enzymes inside. With the artificial cell complete, the interior enzymes made the silicateins, which stuck to antibodies covering the bead’s surface.
Next the scientists triggered a mineral-forming reaction. They broke open the “cells,” soaked them in a solution containing the silicon or titanium molecule used by these proteins, and captured them with a new oil layer.
The silicatein proteins gathered either silicon dioxide or titanium dioxide inside the oil bubble, depending on which mineral precursor they were fed. Then the cells were subjected to two “selection pressures” to weed out non-functional genes and identified those that coded for proteins which made extra strong minerals.
The scientists sorted the beads by size, collecting the largest beads with the thickest mineral layers. Then they shook the beads to break up the mineral coating. Beads that survived this process contained genes for proteins that made minerals of intermediate strength.