UC Irvine

Ornithine decarboxylase (ODC) initiates the synthesis of polyamines (putrescine, spermidine, and spermine) and is highly regulated. We wished to know the importance of the control of ODC synthesis to the rates of growth and polyamine synthesis in the fungus, Neurospora crassa. We identified two control sites of the spe-1 gene, encoding ODC. One was an upstream activation region (UAR) and the other was the DNA encoding the long ODC mRNA leader, which governs polyamine-mediated repression of enzyme synthesis. Transformants receiving copies of spe-1 sequences lacking the UAR compensated for the deficiency by derepression or enzyme stabilization; polyamine synthesis was almost normal. A transformant lacking the spe-1 mRNA leader DNA constitutively expressed ODC mRNA and ODC activity, and synthesized excessive putrescine, especially when provided exogenous ornithine. This transformant grew normally and had only mildly elevated pools of spermidine, the major polyamine of this organism. We conclude that ODC activity normally limits polyamine synthesis, and ornithine becomes limiting in the ODC-constitutive strain. In this strain, however, spermidine synthesis remains rigorously limited by another step of the pathway, as yet unidentified. Thus the control of ODC activity in Neurospora is not vital to growth in laboratory culture, and synthesis of toxic levels of spermidine is limited by other mechanisms.