Templated incorporation hypothesis

Cellulose microfibrils are deposited by cellulose synthases into the cell wall in often strikingly regular patterns. Here we discuss several mechanisms that have been put forward to explain the alignment of cellulose microfibrils that gives rise to ordered cell wall textures the hypothesis that cortical microtubules align cellulose microfibrils during their deposition, the liquid crystal hypothesis in which cellulose microfibrils self-assemble into textures after their deposition, the templated incorporation hypothesis, and the geometrical theory in which the density of active cellulose synthase complexes inside the plasma membrane may dictate the architecture of the cell wall. 

We have formulated a geometrical, mathematical theory for CMF ordering during their deposition, which allows production of axial, helical, crossed, helicoidal, and random wall textures (Emons 1994 Emons and Kieft 1994 Emons and Mulder 1997 1998 2000 2001 2001 Emons et al. 2002 Mulder et al. 2004). Before reviewing our theory, we first discuss the most important alternate CMF ordering hypotheses that have been proposed (1) microtubule-directed CMF orientation, (2) self-assembly like liquid crystals, (3) templated incorporation hypothesis. In addition, we will respond to criticism that has been put forward against the geometrical theory and discuss those predictions from the theory that can be tested experimentally and, therefore, potentially, verify or falsify the theory. 

The preference for the reaction wherein the intermediate compound incorporates the mefa-xylylene spacer—which must arise for its preference to bind the chloride ion—was shown by the extremely efficient formation of the cyclo-phane incorporating two of these moieties that can be prepared in 88% yield in the presence of additional chloride ions. The anions are bound by the imidazolium and meM-xylyl moieties through hydrogen bonds. Competition experiments in the synthesis back up this hypothesis. In addition, careful kinetic studies on the ring closure of an intermediate component prove the template effect. The experiments showed that the rate of the ring closure of the monocationic intermediate to give the dicationic macrocycle is increased up to 10 times in the presence of 40 mM tetrabutylammonium chloride. 

More direct evidence that polymyxin synthesis proceeds by a mechanism that differs from that of protein synthesis comes from experiments with growing cultures of B. polymyxa in which the effects of inhibitors of protein and nucleic acid synthesis on the incorporation of radioactive precursors into protein and polymyxin B were studied (Paulus and Gray, 1964). As shown in Table 7, chloramphenicol, actinomycin D, and to a lesser extent puromycin inhibit the incorporation of L-threonine- C into protein but stimulate its incorporation into polymyxin B. This differential effect of inhibitors on protein and polymyxin synthesis strongly supports the hypothesis that polymyxin synthesis does not proceed by the kind of template mechanism that operates in protein synthesis. 

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