Evolution of asymbiotic nitrogen fixation

Recent observations on the nature of the enzyme complex, nitrogenase, prepared from a variety of nitrogen-fixing micro-organisms, on its substrate specificity, energy requirements, source of reducing power and sensitivity to O₂ now permit speculation on the evolution of biological nitrogen fixation in asymbiotic micro-organisms.Ability to fix N₂ is restricted to procaryotic organisms and is particularly widespread among those having characteristics (e.g. hydrogenase, ferredoxin) regarded as primitive. If the primitive environment was devoid of O₂, the earliest N₂-fixing prokaryote would have been a strict anaerobe, not unlike Clostridium pasteurianum. Yet N₂-fixation seems unnecessary in a primitive ammonia-containing environment, and ammonia represses this function in contemporary species. This apparent paradox, the development of the ability to fix N₂ in circumstances in which it was apparently unnecessary suggests that a substance other than N₂ might have been primary substrate of the primeval enzyme.Substances such as acetylene, cyanide, cyanogen, nitriles or isonitriles are all substrates for nitrogenase and are all probable components of the primitive terrestrial environment. Biologically useful functions which a nitrogenase-like reductase system might have served involving substrates other than N₂ include: (a) a detoxification reaction to nullify the effects of cyanide or cyanogen; (b) a means of generating ATP anaerobically; (c) a hydrogen “escape valve”.Functions (b) and (c) are improbable because they would be physiologically uneconomic; function (a) is plausible.With the emergence of an oxidizing atmosphere, facultative and aerobic N₂-fixing micro-organisms could only retain the nitrogenase system if the O₂-sensitive component was protected from inactivation. In the Azotobacteraceae this is achieved by “conformational protection” together with a high respiration rate; in blue-green algae, a structural compartmentation occurs in the more highly evolved species.     

Morphology of microtubules in rabbit platelets

Summary The morphology of the microtubular wall in rabbit platelets fixed in glutaraldehyde and osmic acid solution and stained with uranyl acetate, or lead hydroxide, or doubly stained, is variable. In cross section, the wall may appear as a uniformly dense annulus, an annulus containing nodular densities about 35 Å in diameter or as a series of contiguous subunits with a circular cross sectional profile about 70 Å wide. The authors relate the varying morphology to the intensity of staining and equate the nodular and circular subunits. Rotational analysis suggests that there are 12 ± 2 subunits in the microtubular wall.We thank Professor A. C. Ritchie for his criticism of the paper and Mrs. M. Lorber and Mrs. M. Mezari for technical assistance. — This work was supported by grants from the Medical Research Council of Canada and the Ontario Heart Foundation.     

A route for direct retinal input to the preoptic hypothalamus: dendritic projections into the optic chiasm.

With the use of Golgi, horseradish peroxidase, and electron microscopic techniques, neurons within a broad region of the preoptic hypothalamus of the mouse were shown to have dendrites that projected well into the depths of the optic chiasm. Further experimental and ultrastructural investigation demonstrated synapses between these dendrites and retinal axonal boutons within the chiasm. All synapses located in the chiasm were classified as Gray's type I. The possible function of these dendritic projections is discussed.