Significance of the amino acid pool in nitrogen metabolism of Penicillium griseofulvum

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Abstract
Conidia of P. griseofulvum were incubated in full and N-free media, and changes in the amino-acid pool were followed. Utilization of free amino acids present in dormant conidia, especially glutamine and y -aminobutyric acid, occurred at an early stage in both media, although an external N source was required before visible signs of normal germination could be detected. Net utilization of over 90% of the initial amino-acid pool in the synthesis of hot-water-insoluble nitrogenous components of P. griseofulvum was demonstrable when the mycelium was transferred to N free medium at pH 3. 6. At pH 6.0 this process became masked by the onset of net protein degradation to amino acids and peptides. At pH3.0 both synthetic and degradative systems were inhibited. A study of incorporation of N15 -labelled ammonia revealed that almost all of the free amino acid fraction of the mycelium was involved in the assimilation of inorganic N, and that at least 67% of the assimilated N passed through this fraction. Incorporation into glutamic acid was sufficient to account for the synthesis of at least 85% of the total mycelial -amino N content. The redistribution of label observed after the exhaustion of exogenous N confirmed previous nonisotopic evidence that a rapid turnover of insoluble N, involving breakdown to the amino-acid level, occurred during N starvation. Extracellular organic N released after exhaustion of ammonia arose from the breakdown of insoluble mycelial material.