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ParM is a prokaryotic actin homologue which provides the force to drive copies of the R1 plasmid to opposite ends of rod shaped bacteria before cytokinesis. ParM is a monomer that is encoded in the DNA of the R1 plasmid and manufactured by the host cell’s ribosomes. In the cytoplasm it spontaneously polymerizes forming short strands that either bind to ParR or hydrolyze. ParR stabilizes ParM and prevents it from hydrolyzing. Once bound by ParR at both ends, monomer units continue to attach to the ends of the ParM and the resulting reaction pushes R1 plasmids to opposite ends of the cell. ParMs from different bacterial plasmids can form astonishingly diverse helical structures comprising two or four strands to maintain faithful plasmid inheritance. == Action == In vitro the ParM monomer has been observed polymerizing both with ATP and with GTP, but experiments by Popp et al. seem to indicate that the reaction “prefers” GTP and that GTP is the nucleotide that most likely makes the significant contributions in the cell.〔 For the remainder of this article GTP will be assumed to be the active nucleotide although many experiments have used ATP instead. ParM binds and hydrolyzes GTP as it polymerizes. The current dominant belief is that a “cap” of GTP is required at the ends of the ParM polymer strands to prevent them from hydrolyzing. Although GTP is hydrolyzed by the ParM units after attachment, it is believed that the energy that drives the plasmids is derived from the Gibbs free energy of the ParM monomer concentrations, and not the energy released from GTP hydrolysis. The concentrations of ParM monomer and polymer must be kept out of equilibrium at the ends where attachment is occurring for the reaction to proceed regardless of GTP concentrations. Once the ParM has pushed plasmids to opposite ends of the cell the polymer rapidly depolymerizes—returning the monomer units to the cytoplasm. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ParM」の詳細全文を読む スポンサード リンク
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