Arginine synthesis and urea formation via the ornithine cycle occurred in sporophore cap and stipe of Although urease was found at high activity in mycelium and stipe, it was not detectable in extracts of cap tissue, but arginine biosynthesis was specifically amplified during development of the cap as judged from metabolism of isotopically labelled substrates and increased enzyme activities. Four enzymes, NADP-linked glutamate dehydrogenase, glutamine synthetase, ornithine acetyltransferase and ornithine carbamoyl-transferase, were considerably derepressed in developing caps while remaining low (or declining) in activity in the stipes supporting those caps. Co-ordinate regulation of these enzymes could also be demonstrated in vegetative mycelium subjected to particular synthetic growth media. Arginine, alanine and glutamate accumulated in the cap as a result of amplified arginine biosynthesis. A greater than twofold increase in the quantity of urea in the cap was also evident during development, although the concentration of urea remained essentially unchanged. A causal relationship between urea accumulation and water influx into developing cap tissues is suggested. The inflation of the gill hymenium cells accounts for the `umbrella-like' cap expansion that characterizes spp.


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