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One Microbe as a Group of 200 Protein Machines

Last post 12-01-2009 9:07 PM by cartoon. 0 replies.
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  • 12-01-2009 9:07 PM

    One Microbe as a Group of 200 Protein Machines


    Molecular biology for years meant breaking down living cells to their smallest component parts, the genes and proteins that govern what a cell does. But a list of parts tells only so much. To understand how living cells really work, biologists are now trying to visualize how the parts are assembled into operational units.

    A team of European scientists has chosen one of the smallest known bacteria, called Mycoplasma pneumoniae, as a test-bed for trying to integrate all the bottom-up knowledge about an organism into a full understanding of how it actually works. The microbe causes a form of bacterial pneumonia and has shed so many functions from its stripped down genome that it can survive only as a parasite on other cells.

    The European findings so far, reported in the current Science, are that the bacterium is a collection of some 200 specialized protein machines.

    The machines are composed of individual proteins, which recognize each other and assemble into complexes. Some of the machines make copies of the genes embodied in the DNA of the bacterium’s genome. Others, called ribosomes, synthesize proteins according to the genetic instructions they receive. Another class, called chaperones, make sure the new proteins fold up correctly. Then there are processing machines in which each component carries out one step of a multistage chemical process.

    Developing a systematic view of a cell’s organization has been possible only since methods for decoding genomes. With the genome in hand, biologists can identify all proteins a cell produces and trace the pattern of interactions among the various kinds.

    The European team, lead by Anne-Claude Gavin at the European Molecular biology Laboratory in Heidelberg, Germany, chose M. pneumoniae as their subject because it has a mere 689 protein-coding genes, compared with the 2,000 or so of other bacterial species. By tagging each protein in turn and noting which others it had a natural affinity for, the team discovered that the 689 proteins served as component parts for some 200 different machines.

    But this is only a slice of the bacterium’s full complexity. The set of 200 machines is referred to as protein repertoire or proteome. The bacterium’s genome can produce many versions of its proteome, each designed to aid survival in a different environment. “There as many proteomes as you have external conditions,” Dr. Gavin said.

    Governing the production of these proteomes is a genetic control system that is far more complex than had been expected for a small bacterium, a second European team reports in Science.

    The minute examination of this tiny bacterium is intended as a step toward understanding it as a system, which requires cataloging all its components and their interactions.

    In a commentary in Science, two biologists at the University of Arizona, Howard Ochman and Rahul Raghavan, say the European work shows that “there is no such thing as a ‘simple’ bacterium,” given how complex even this miniature member of the bacterial world has turned out to be.

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