Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [82]
Scientists are now developing hypotheses from this evidence to explain how flagella evolved. Pallen and Nicholas Matzke, now a graduate student at the University of California, Berkeley, offered one hypothesis in 2006. Before there were flagella, Pallen and Matzke argued, there were simpler parts carrying out other functions. Gene duplication made extra copies of those parts, and mutations caused the copies to be combined into the evolving flagellum. Today flagella serve one main function: to swim. But their parts did not start out that way.
The flagellum’s syringe may have begun as a simple pore that allowed molecules to slip through the inner membrane. A proton-driving motor became linked to it, allowing it to push out big molecules. This primitive system may have allowed ancient bacteria to release signals or toxins. Two kinds of structures eventually evolved from it: the type III secretion system and the needle that injects pieces of the flagellum across the membrane.
The next step in the evolution of flagella may have come when the needle began squirting out sticky proteins. Instead of floating away, these proteins clumped around the pore. Bacteria could have used these sticky proteins as many species do today, to allow them to grip surfaces. The microbes added more proteins to produce hairs, which could reach out farther to find purchase.
In the next step, this sticky hair began to move. A second type of motor became linked to it, which could make the hair quiver. Now the microbe could move. Its crude, random movement may have allowed it to disperse during times of stress. Over time this protoflagellum became fine-tuned. Gene duplication allowed the proteins making up the filament to become a flexible hook at the base and stiff, twisted fibers along the shaft. And finally bacteria began to steer. One of their chemical sensing systems became linked to their flagella, allowing them to change their direction.
This hypothesis is not the unveiling of absolute truth. Scientists don’t have that power. What scientists can do is create hypotheses consistent with previous observations—in this case, observations of the variations in flagella, the components that play other roles in bacteria, and the ways in which evolution combines genes for new functions. Pallen and Matzke’s hypothesis may well prove to be flawed, but the only way to find out is to search the genomes of E. coli and other microbes for more clues as to how the flagellum was assembled, to study how intermediate structures work, and perhaps even to genetically engineer some of the intermediate steps that have disappeared. A better hypothesis may emerge along the way. But it is a far superior hypothesis to one built on nothing but appearances and a personal sense of disbelief.
NETWORKS UNDER CONSTRUCTION
In order to build a flagellum, E. coli does not simply churn out all the proteins in a blind rush. It controls the construction with a sophisticated network of genes. Only when it detects signs of stress does it switch on the flagella-building genes, and it uses a noise filter to avoid false alarms. It turns the genes on step by step as it gradually builds up the flagellum, then it turns them off. And like the flagellum, E. coli’s control networks have an ancient history of their own.
In 2006, M. Madan Babu, a biologist at the University of Cambridge, and his colleagues published a major investigation of how E. coli’s circuitry evolved. They began by searching for E. coli’s genetic switches—the proteins that grab on to DNA and turn on, turn off,