In the Company of Microbes. Moselio Schaechter
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March 11, 2013
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#67
by Elio
Richard Feynman, the famous physicist, said: It is very easy to answer many of these fundamental biological questions; you just look at the thing! To take him up on it, imagine a microscope that lets you observe single molecules in a living cell at one Angström resolution. What’s the first thing you would do with it?
October 28, 2010
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10
Self-Assembly for Me
by Elio
I have the grating feeling that the subject of self-assembly of complex biological structures may not always amass the level of respect it deserves. I reckon that its importance is generally appreciated but, as topics go, it tends at times to be set aside. Yet, this is one of the most magnificent aspects of biology, one that beautifully combines logic with mechanics and attests forcibly to the power of evolution. And it goes back a ways. The pioneering study on the self-assembly of phages played an integral role in the development of molecular biology.
Today, the assembly of the bacterial flagellar motor rates high on the list of exciting self-assembly phenomena, possibly vying with that of viral structure. The motor is a key constituent of bacterial flagella. It is located at the base of the structure and is responsible both for anchoring it to the bacterium and providing the mechanism for its rotation. It is a structure with many components, and its assembly constitutes an amazing engineering feat. One of the earliest indications of its complexity was recently exposed in these pages. Going back to 1971, purified flagella were convincingly shown to have an intricate base, consisting of several rings presumed to anchor the flagellum to the bacterial envelopes in a rotor-stator arrangement. This structural design for a molecular machine delightfully explained how flagella could both rotate and be kept in place.
Time passed since this spectacular early imagery, and with it came the development of techniques of previously unimaginable power.
Flagellar motor structures obtained by electron cryotomography and subtomogram averaging. Left column 20-nm thick central slices through tomograms of individual cells exhibiting flagellar motors, arranged in the same order as they appear on a phylogenetic tree. Scale bar, 50 nm. Right column Axial slices through average reconstructions of each motor. Scale bar, 10 nm.
Source: Chen S, Beeby M, Murphy GE, Leadbetter JR, Hendrixson DR, Briegel A, Li Z, Shi J, Tocheva EI, Müller A. 2011. Structural diversity of bacterial flagellar motors. The EMBO 30:2972-2981.
High among these is electron cryotomography, a way to reveal the 3D arrangements in unfixed biological material under the electron microscope. This is somewhat analogous to a CAT scan, but instead of the optical sections being parallel, they are produced by tilting the specimen at various angles. With this technique, along with the computerized analysis of single images (electron cryotomography and subtomography averaging), one can observe structures at ‘macromolecular’ (several nanometer) resolution. In other words, in exquisite molecular detail. The elements of the flagellar motor, the various rings, the center rod, the stator component, and what is known as the export apparatus, are now revealed in glorious detail. It’s like looking at the wheel assembly of a car reduced about 10 million-fold.