الفهرس 
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Abstract
In all biological systems the assimilation of nitrogen into macromolecu7les is essential for growth. The metabolic pathways of nitrogen metabolism can be divided into two classes : The assimilatory pathways mecessary for the utilization of nitrogen from compounds available in the medium, and the biosynthetic pathways leading to the production of the nitrogen-containing compounds of the cell. The specific steps in these pathways vary with the organism (T?Tyler, 1978). A fair description of nitrogen assimilation has been achieved only in very few systems but by far the most comprehensive picture available up to date is in the enteric bacteria namely rscherichia coli, salmonella typthimurium, klebsieila pneumoniae and K. aweogenes (Magasanik, 1982; Merrick, 1987) .