Expression of the human spermidine/spermine N1-acetyltransferase in spermidine acetylation-deficient Escherichia coli.

A cDNA encoding the human spermidine/spermine N1-acetyltransferase (N1SSAT) was conditionally expressed in a strain of Escherichia coli deficient in spermidine-acetylating activity. Conditional expression of this cDNA was performed under the control of the lac promoter, by addition of the non-hydrolysable lactose analogue isopropyl beta-D-thiogalactoside. Expression of the N1SSAT cDNA oriented in the sense direction resulted in the acetylation of spermidine at the N1 but not the N8 position and a decrease in endogenous spermidine contents and growth rates in these bacteria. When this cDNA was expressed in the antisense orientation, spermidine acetylation was not detected and endogenous spermidine contents and growth rates were unaffected. Increasing the endogenous N1-acetylspermidine concentration by addition of this amine to the culture medium did not suppress growth, and increasing endogenous spermidine pools by exogenous addition was not sufficient to restore optimal growth in cells expressing the human N1SSAT. Exogenous spermidine, but neither N1- nor N8-acetylspermidine, stimulated cell growth in strains unable to synthesize spermidine. These results suggest that one physiological consequence of spermidine acetylation in E. coli is growth inhibition. The mechanism of this inhibition seems to involve the formation of acetylspermidine, and is not simply due to a decrease in the intracellular concentration of non-acetylated spermidine.