Polyamine modulon in yeast-Stimulation of COX4 synthesis by spermidine at the level of translation.

We proposed that a group of genes whose expression is enhanced by polyamines at the level of translation in Escherichia coli and mammalian cells be referred to as a "polyamine modulon". In Saccharomyces cerevisiae, proteins whose synthesis is enhanced by polyamines at the level of translation were searched for using a polyamine-requiring mutant of S. cerevisiae deficient in ornithine decarboxylase (YPH499 Deltaspe1). Addition of spermidine to the medium recovered the spermidine content and enhanced cell growth of the YPH499 Deltaspe1 mutant by 3-5-fold. Under these conditions, synthesis of COX4, one of the subunits of cytochrome C oxidase (complex IV), was enhanced by polyamines about 2.5-fold at the level of translation. Accordingly, the COX4 gene is the first member of a polyamine modulon in yeast. Polyamines enhanced COX4 synthesis through stimulation of the ribosome shunting of the stem-loop structures (hairpin structures) during the scanning of the 5'-untranslated region (5'-UTR) of COX4 mRNA by 40S ribosomal subunit-Met-tRNA(i) complex.