Involved in the protection against polyamine toxicity by regulating their concentration (PubMed:7642535, PubMed:10986239). Catalyzes the transfer of an acetyl group from acetyl coenzyme A (AcCoA) to the primary amino groups of spermidine to yield N(1)- and N(8)-acetylspermidine (PubMed:8077207, PubMed:7052085, PubMed:6297970). It can also use spermine, but not putrescine (PubMed:7052085). In addition, may act as a modulator of transcription through its ability to interact with the two-component response regulator RcsB. Inhibits transcription of the small RNA regulator rprA in an acetylation-independent manner. Interaction with the DNA-binding domain of RcsB might be responsible for SpeG-dependent inhibition of RcsB-dependent rprA transcription (PubMed:30562360). SpeG does not acetylate RcsB in vitro (PubMed:30562360). acetyl-CoA + an alkane-alpha,omega-diamine = an N-acetylalkane-alpha,omega-diamine + CoA + H(+) acetyl-CoA + spermidine = CoA + H(+) + N(1)-acetylspermidine acetyl-CoA + spermidine = CoA + H(+) + N(8)-acetylspermidine acetyl-CoA + spermine = CoA + H(+) + N(1)-acetylspermine Heat shock, alkaline shift, ethanol treatment and poor nutrient availability produce increased concentrations of monoacetylated spermidine (PubMed:2061318). Inhibited by alkaline phosphatase, N(1),N(8)-bis(ethyl)spermidine (BESPD) and N(1),N(12)-bis(ethyl)spermidine (BESPM) (PubMed:8077207, PubMed:6297970). The level of spermidine acetyltransferase activity increases at low temperature to prevent spermidine toxicity. Amine and polyamine degradation; spermidine degradation. Amine and polyamine degradation; spermine degradation. Homododecamer (PubMed:31205017). Ligand-free SpeG forms closed symmetric dodecamers in the crystal and open asymmetric dodecamers in solution (PubMed:31205017). In the absence of ligand, also exists in solution as hexamers and tetramers, but not dimers (PubMed:23908034, PubMed:8077207, PubMed:31205017). Can interact with the DNA-binding carboxyl domain of RcsB in the presence or absence of spermidine (PubMed:30562360). Constitutively expressed throughout log and plateau phases of growth. Cells lacking this gene accumulate spermidine at late stationary phase and show a reduced cell viability due to a decrease in protein biosynthesis (PubMed:7642535). At 37 degrees Celsius, growth of mutant is normal in the presence of 0.5 or 1 mM spermidine. However, following a shift to 7 degrees Celsius, the addition of 0.5 or 1 mM spermidine results in inhibition of cellular growth or cell lysis, respectively. Furthermore, at 7 degrees Celsius, spermidine accumulation resulted in a decrease in total protein synthesis accompanied by an increase in the synthesis of the major cold shock proteins CspA, CspB, and CspG (PubMed:10986239). Acetylation neutralizes the charge of the polyamine, which is then typically excreted from the cell. Belongs to the acetyltransferase family.