EXPRESSION OF THE HUMAN SPERMIDINE SPERMINE N-1-ACETYLTRANSFERASE IN SPERMIDINE ACETYLATION-DEFICIENT ESCHERICHIA-COLI/

A cDNA encoding the human spermidine/spermine N-1-acetyltransferase (N (1)SSAT) was conditionally expressed in a strain of Escherichia coli d eficient in spermidine-acetylating activity. Conditional expression of this cDNA was performed under the control of the lac promoter, by add ition of the non-hydrolysable lactose analogue isopropyl beta-D-thioga lactoside. Expression of the N(1)SSAT cDNA oriented in the sense direc tion resulted in the acetylation of spermidine at the N-1 but not the N-8 position and a decrease in endogenous spermidine contents and grow th rates in these bacteria. When this cDMA was expressed in the antise nse orientation, spermidine acetylation was not detected and endogenou s spermidine contents and growth rates were unaffected. Increasing the endogenous N-1-acetylspermidine concentration by addition of this ami ne to the culture medium did not suppress growth, and increasing endog enous spermidine pools by exogenous addition was not sufficient to res tore optimal growth in cells expressing the human N(1)SSAT. Exogenous spermidine, but neither N-1- nor N-8-acetylspermidine, stimulated cell growth in strains unable to synthesize spermidine. These results sugg est 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 spermidi ne.