Researchers at the University of Cambridge have uncovered the mechanism by which bacteria build their surface propellers (flagella). The results, published in the November 10, 2013 online edition of Nature (DOI:10.1038/nature12682), demonstrate how the mechanism is powered by the subunits of the flagella themselves as they link in a chain that is pulled to the flagellum tip.

Previously, scientists thought that the building blocks for flagella were either pushed or diffused from the flagellum base through a central channel in the structure to assemble at the flagellum tip, which is located far outside the cell. However, these theories are incompatible with recent research that shows that flagella grow at a constant rate. The unexpected chain mechanism, in which subunits linked in a chain pull themselves through the flagellum, transforms current understanding of how flagellum assembly is energized.

The research team, led by Gillian Fraser and Colin Hughes, found that as each flagellum “nanomachine” is assembled, thousands of subunit building blocks are made in the cell and are then unfolded and exported across the cell membrane. Like other processes inside cells, this initial export phase consumes chemical energy. However, when subunits pass out of the cell into the narrow channel at the center of the growing flagellum, there is no conventional energy source and they must somehow find the energy to reach the tip.

The research team has shown that, at the base of the flagellum, subunits connect by head-to-tail linkage into a long chain. The chain is pulled through the entire length of the flagellum channel by the entropic force of the unfolded subunits themselves. This produces tension in the subunit chain, which increases as each subunit refolds and incorporates into the tip of the growing structure. This pulling force automatically adjusts with increasing flagellum length, providing a constant rate of subunit delivery to the assembly site at the tip.

Co-researcher Eugene Terentjev, of the Cavendish Laboratory, said, “Understanding how polymers move through channels is a fundamental physical problem. Gaining insight into this [research on bacteria] has potential applications in other disciplines, for instance in nanotechnology, specifically the building of new nanomaterials.”