Polyamine Regulation of Ornithine Decarboxylase Synthesis in Neurospora crassa

Abstract

ABSTRACT Ornithine decarboxylase (ODC) of the fungus Neurospora crassa , encoded by the spe-1 gene, catalyzes an initial and rate-limiting step in polyamine biosynthesis and is highly regulated by polyamines. In N. crassa , polyamines repress the synthesis and increase the degradation of ODC protein. Changes in the rate of ODC synthesis correlate with similar changes in the abundance of spe-1 mRNA. We identify two sequence elements, one in each of the 5′ and 3′ regions of the spe-1 gene of N. crassa , required for this polyamine-mediated regulation. A 5′ polyamine-responsive region (5′ PRR) comprises DNA sequences both in the upstream untranscribed region and in the long 5′ untranslated region (5′-UTR) of the gene. The 5′ PRR is sufficient to confer polyamine regulation to a downstream, heterologous coding region. Use of the β-tubulin promoter to drive the expression of various portions of the spe-1 transcribed region revealed a 3′ polyamine-responsive region (3′ PRR) downstream of the coding region. Neither changes in cellular polyamine status nor deletion of sequences in the 5′-UTR alters the half-life of spe-1 mRNA. Sequences in the spe-1 5′-UTR also impede the translation of a heterologous coding region, and polyamine starvation partially relieves this impediment. The results show that N. crassa uses a unique combination of polyamine-mediated transcriptional and translational control mechanisms to regulate ODC synthesis.