## 1 Introduction  
Polyamines, which are thought to be present at mM concentrations in most growing cells, are essential for cell growth and viability [1]. Polyamines, such as putrescine (PUT) and spermidine (SPD), are distributed ubiquitously across all domains of life. Intracellular polyamine concentrations are strictly regulated through biosynthesis, degradation, uptake, and excretion, since depletion or excessive accumulation of polyamines causes cell growth inhibition and lower cell viability [2, 3]. In mammalian cells, the main polyamines consist of SPD and spermine (SPM). Cytotoxic products, such as acrolein, hydrogen peroxide, aminoaldehydes, and ammonia, are produced by the oxidation of SPM during polyamine catabolism. Among them, acrolein is a highly toxic unsaturated aldehyde and has been shown to inhibit cell growth [4]. However, in many mesophilic bacteria, the main polyamines consist of PUT and SPD, but not SPM [5], so acrolein cannot be produced by the oxidation of polyamines due to the lack of a polyamine oxidase gene. Therefore, the cytotoxic mechanism that results from accumulation of polyamines in bacteria has not been characterized in detail. Because polyamines exist mostly as polyamine-RNA complexes in cells [6], their effects on cell growth and viability are presumed to be caused by the changes in RNA functions. We previously reported that polyamines stimulate general protein synthesis in vitro, increase the fidelity of translation [7; 8], and induce the assembly of 30S ribosomal subunits in vivo [9; 10]. In addition, we reported that polyamines enhanced synthesis of the 20 specific proteins important for cell growth and viability at the level of translation in _Escherichia coli_[11; 12; 13]. The genes encoding these 20 different proteins were termed as the \"polyamine modulon\". There are three different mechanisms underlying stimulation of protein synthesis from the genes encoded by polyamine modulon. Firstly, polyamines stimulate protein synthesis when the Shine-Dalgarno (SD) sequence in the mRNA is obscure or is distant from initiation codon AUG. Polyamines cause structural changes of a region of the SD sequence and initiation codon AUG, then facilitate formation of the initiation complex. In particular, the structural change of the bulged-out region of a hairpin structure of 5'-untranslated regions by polyamines is important for polyamine stimulation of these protein syntheses [14]. This is the case for _cpxR_, _emrR_, _fecl_ (\(\sigma^{18}\)), _fs_, _hns_, _oppA_, _rmf_, _rpoE_ (\(\sigma^{24}\)), _rpoN_ (\(\sigma^{54}\)), _rpoZ_ (\(\omega\)), _soxR_, and _stpA_. Secondly, polyamines enhance the inefficient initiation codon UUG- or GUG-dependent fMet-tRNA binding to ribosomes. This is the case for _cra_, _cva_, _frr_, _gshA_, _spoT_, and _uvY_. Thirdly, polyamines stimulate read-through of amber codon UAG on ribosome-associated _rpoS_ (\(\sigma^{38}\)) mRNA by facilitating the binding of Gln-tRNA\({}^{\rm supE}\), or stimulating a +1 frameshift at the 26 th UGA codon of the _prfB_ mRNA encoding RF2. The intracellular content of polyamines, such as PUT and SPD, in _E. coli_ is elaborately regulated by biosynthesis, degradation, and transport. Among these, SPD acetyltransferasease is a key catabolic enzyme in controlling polyamine concentrations. This enzyme is not required for cell growth in _E. coli_ cells, but is involved in the responses to a variety of chemical and physical stresses [15; 16]. It has also been reported that polyamine catabolism is a core metabolic response to several stresses [17]. These reports indicate that polyamine metabolism is important for stress response, while we previously reported that SPD toxicity is increased in response to excessive accumulation of SPD when SPD acetyltransferase is lacking [18].