On the cytostatic mechanism of cyclophosphamide. Inhibition of aminoacylation of transfer RNA and induction of stringent control in Escherichia coli by 4-hydroperoxycyclophosphamide.
4-Hydroxycyclophosphamide, a highly cytotoxic derivative of cyclophosphamide, has bacteriostatic properties at concentrations higher than 80 microM when given to Escherichia coli growing in glucose/salt medium. The inhibitory effect on growth, protein and RNA syntheses is relieved by the addition of a mixture of amino acids, with leucine being obligatory. In cells treated with the drug, aminoacylation of tRNALeu is drastically decreased, but the content of free leucine is not. It is concluded that 4-hydroxycyclophosphamide interferes with the charging process of tRNALeu in E. coli leading to an inhibition of protein synthesis. The concomitant inhibition of RNA synthesis can adequately by explained by the stringent response phenomena. In a relaxed mutant strain (rel) of E. coli, protein synthesis is also inhibited, but the accumulation of RNA continues after the addition of the drug. Guanosine tetraphosphate (ppGpp), the putative mediator of the stringent control, accumulates in drug-treated stringently controlled E coli but not in the relaxed mutant. An additional direct effect of the drug on RNA synthesis can therefore be ruled out.