Evolutionary Remarks

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

RCII may subsequently have been transformed into RCI by formation of the Fx cluster and eventually the capturing of a soluble 2[4Fe-4S] protein as an RC-associated subunit. These additions would have allowed electrons to leave the space of the membrane and serve for reductive processes in the dark reactions of photosynthesis. Our present knowledge concerning distribution of HiPIPs among species indicate that this electron carrier would have been invented only lately within the branch of the proteobacteria. Tbe evolutionary driving 

We thank P. Bertrand, M. Bruschi, F. Guerlesquin, B. Guigliarelli, R. Kappl, M. Hippier, J. Htlttermann, D. Lemesle-Meunier, U. Mtlhlenhoff, A. W. Rutherford, C. L. Schmidt, A. Schricker, and P. S6tif for stimulating discussions and for communicating results prior to publication. 

Feiler, U. Hauska, G. In Anoxygenic Photosynthetic Bacteria Blankenship, R. E. Madigan, M. M. and Bauer, C. E., Eds. Kluwer Acad. Publ. Dordrecht, 1995 pp. 665-685. 

Schricker, A. 1997, Soluble electron transport proteins in Heliohacillus mohilis, Diploma Thesis, University of Freiburg/FRG. 

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The most dramatic illustration of the evolutionary conservation of homeobox genes comes from experiments that swap Drosophila and mammalian cognates and test for functional equivalence. McGinnis and co-workers have reported remarkable phenotypic similarities in flies that misexpress Drosophila homeobox genes or their mammalian counterparts. Indeed, this ectopic expression assay in Drosophila indicates that the Drosophila and cognate mammalian homeobox proteins are essentially functionally identical. 

The processes of evolution have been remarkably successful in creating animals that are well adapted to their environment. By continually adjusting the population through genetic mutation, evolutionary change works toward the perfect individual to fill a particular environmental niche. Not every new individual is well suited to its environment, of course, and the fitness of new individuals is repeatedly tested as the animals try to survive, prosper, and reproduce. 

The protein NompC is a TRP channel and appears to be the major transduction channel in fly bristles [6], Bristle mechanotransduction resembles hair-cell transduction remarkably in its speed, polarity and adaptation [6], suggesting the possibility of a close evolutionary relationship between these mechanoreceptors. NompC is not the only... 

In Chapter 3, it was noted that remarkably few biochemical pathways lead from Basic Integrated Metabolism into the pathways that serve to produce substances with Type 11 and Type 111 properties. Using the concepts discussed, it is possible to offer one evolutionary scenario to explain why a few, rather than many pathways flow from Basic Integrated Metabolism. As in the case of most evolutionary arguments, it is not difficult to produce a credible scenario but hard to find supporting evidence simply because the postulated events would have occurred early in the evolution of life and there are no known biochemical fossils . However, some computer modelling has at least provided some support for the approach adopted. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

Metallothioneins from Neurospora crassa contain only 25 amino acid residues, but the primary structure is quite similar to the N-terminal part of the mammalian proteins.1454 These data indicate that the gene that codes for the Neurospora crassa metallothionein is evolutionary related to the gene of the vertebrate metallothioneins. Equine metallothionein exists in two major variants (metallothionein 1A and metallothionein 2A), which show remarkable similarities. Some allelic polymorphic variants also occur in man, horse and rabbit. Recent developments have been comprehensively reviewed.14678... 

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