Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [23]
When scientists map the pathways that a carbon atom can take through E. coli’s metabolism, the picture they see looks like a bow tie. On one side of the bow tie are the chemical reactions that draw in food and break it down. These reactions follow each other along simple pathways, a fan of incoming arrows. Eventually the arrows all converge on the bow tie’s knot. There the pathways get much more complicated. The product of a reaction may get pulled into many different reactions, depending on the conditions at that moment. It is there, in the knot, that E. coli creates the building blocks for all its molecules. The building blocks enter the other side of the bow tie—an outgoing fan of pathways. Each pathway produces a very different sort of molecule—this one a membrane molecule, that one a piece of RNA, another one a protein. The pathways on the far side of the bow tie fan out without crossing over. A molecule on its way to becoming a protein does not become a piece of DNA.
The bow tie architecture in E. coli makes good engineering sense. Man-made networks, such as a telephone network or a power grid, are often laid out in a bow tie as well. A bow tie architecture lets networks run efficiently and robustly. The Internet, for example, has an incoming fan made up of signals from e-mail programs, Web browsers, and all sorts of other software, each with its own peculiar sorts of information processing. In order for this stream of data to get onto the Internet, it must first be turned into a code that obeys the Internet’s protocols. These data streams move from personal computers to servers and then into a small core of routers. The signals can then flow into an outgoing fan of pathways, toward another computer, where the standard stream of data can be converted into a picture, a document, or some other peculiar form.
In both the Internet and E. coli, the bow tie knot allows each network to function even when parts of it fail. A mutation that destroys one metabolic reaction will not kill E. coli because in the knot there are other pathways onto which it can still shunt carbon. The Internet can continue sending messages even after one of the servers shuts down because it can move the messages through another pathway.
The bow tie architecture also saves energy in both systems. If E. coli did not have a bow tie, it would have to create a dedicated pathway of enzymes to make every molecule it needed. Each of those enzymes would require its own gene. Instead, E. coli’s pathways all dump their products into the same network in the knot of the bow tie. Likewise, the Internet does not have to link every computer directly to every other one, or use special codes for every kind of file it carries. In both cases this arrangement is possible only because the entire network obeys certain rules. On the Internet every message must be converted into the same data packets. In E. coli all energy transfers must use the same currency: ATP.
The inventors of the Internet did not realize they were creating this kind of network. They were only trying to balance cost and speed as they joined servers together. But unintentionally they created a model of E. coli that spans the Earth.
VIVE LA DIFFÉRENCE
We all have our own tastes. I don’t understand why some people eat snails. I can’t say for sure why I dislike them, but I can certainly think up a few stories. Maybe I have a certain kind of sensor on the cells of my tongue that goes into a spasm of dismay. Or maybe some network of neurons in my brain associates the taste of snails with some awful memory from my distant past. Or maybe I simply never had the opportunity to come to love snails because I grew up eating pizza and hamburgers and peanut butter. The gastronomic window has now closed.
I have no way of knowing whether any of those possibilities is true. I can’t go back