Regulation of polyamine synthesis in relation to putrescine and spermidine pools in Neurospora crassa.

Abstract

Polyamine pools were measured under various conditions of high and low concentrations of cytosolic ornithine with the wild-type and mutant strains of N. crassa. In minimal medium, the wild-type strain had 1-2 nmol putrescine and .apprx. 14 nmol spermidine/mg (dry wt); no spermine occurred in N. crassa. Exogenous ornithine caused a rapid but quickly damped increase in the rate of polyamine synthesis. This effect was greater in a mutant (ota) unable to catabolize ornithine. No turnover of polyamines was detected during exponential growth. Exogenous spermidine was not taken up efficiently by N. crassa; the compound could not be used directly in studies of regulation. By nutritional manipulation of a mutant strain, aga, lacking arginase, cultures were starved for ornithine and ultimately for putrescine and spermidine. During ornithine starvation, the remaining putrescine pool was not converted to spermidine. The pattern of polyamine synthesis after restoration of ornithine to the polyamine-deprived aga strain indicated that, in vivo, spermidine regulated polyamine synthesis at the ornithine decarboxylase reaction. The regulatory process was a form of negative control which became highly effective when spermidine exceeded its normal level. The relationship between the regulation of polyamine synthesis and the ratio of free to bound spermidine was discussed.