The Polyamine Modulon: Genes Encoding Proteins Whose Synthesis Is Enhanced by Polyamines at the Level of Translation

Abstract

Polyamines mainly exist as an RNA–polyamine complex in cells. Thus, the effects of polyamines on protein synthesis were examined in Escherichia coli, Saccharomyces cerevisiae, and mouse mammary carcinoma cells. It was found that several kinds of protein synthesis, which are involved in cell growth and viability, were stimulated by polyamines at the level of translation. We proposed that a set of genes whose expression is enhanced by polyamines at the level of translation can be classified as a “polyamine modulon.” Thus far, 17 kinds of genes in E. coli and 4 kinds of genes in eukaryotes were identified as members of polyamine modulon. There are several mechanisms underlying polyamine stimulation of protein synthesis in E. coli. First, polyamines stimulated several kinds of protein synthesis when a Shine–Dalgarno (SD) sequence in the mRNAs is distant from the initiation codon AUG. Second, polyamines stimulated an inefficient initiation codon UUG- or GUG-dependent fMet-tRNA binding to ribosomes. Third, polyamines stimulated read-through of the amber codon UAG by Gln-tRNA^SupE or +1 frameshifting at the termination codon UGA on the open reading frame. In eukaryotes, polyamines stimulated ribosome shunting during the scanning of the Met-tRNA_i-40S ribosomal subunit complex from the cap structure to the initiation codon AUG of mRNAs.