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One of the most characteristic structural features of the prokaryotic 50S ribosomal subunit is the "stalk", a protuberance that contains the protein called L7/L12. In 'E. Coli', L7/L12 is a dimeric protein which is able to undergo rapid subunit exchange (Hamman et al., 1996 Biochemistry 35: 16680-6,). Recent X-ray data reported a tetrameric arrangement of the L12 proteins isolated from 'Termotoga Maritima'. This structure was used to propose 2 alternative dimerization modes. In one mode, the 2 monomers of L12 form a tight symmetric and parallel dimer held together by a four helix bundle. In the other mode, the two monomers bind through their N-terminal region in an anti-parallel configuration with one monomer in a tight conformation and the other monomer adopting an elongated shape. The first mode of dimerization has been used to represent L7/L12 into the structural model of the 70S ribosome and a recent publication (Nomura et al., 2003 Biochemistry 42: 4691-8) used this mode to discuss their experimental results. In this work, we present data from Forster Resonance Energy Transfer experiments using labelled cysteine mutants of 'T. maritima', which support the second mode of dimerization, i.e. in an anti-parallel orientation and an elongated configuration. We also demonstrate that the rate of subunit exchange among a population of 'T. maritima' LI2 is significantly slower than in the 'E. coli' system. |
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