The molecular basis of leucine auxotrophy of quinone-treated Escherichia coli. Active site-directed modification of leucyl-tRNA synthetase by 6-amino-7-chloro-5,8-dioxoquinoline

Comparison of two ecotypes of the metal hyperaccumulator Thlaspi caerulescens (J. & C. PRESL) at the transcriptional level

This paper investigates differences in gene expression among the two Thlaspi caerulescens ecotypes La Calamine (LC) and Lellingen (LE) that have been shown to differ in metal tolerance and metal uptake. LC originates from a metalliferous soil and tolerates higher metal concentrations than LE which originates from a non-metalliferous soil. The two ecotypes were treated with different levels of zinc in solution culture, and differences in gene expression were assessed through application of a cDNA microarray consisting of 1,700 root and 2,700 shoot cDNAs. Hybridisation of root and shoot cDNA from the two ecotypes revealed a total of 257 differentially expressed genes. The regulation of selected genes was verified by quantitative reverse transcriptase polymerase chain reaction. Comparison of the expression profiles of the two ecotypes suggests that LC has a higher capacity to cope with reactive oxygen species and to avoid the formation of peroxynitrite. Furthermore, increased transcripts for the genes encoding for water channel proteins could explain the higher Zn tolerance of LC compared to LE. The higher Zn tolerance of LC was reflected by a lower expression of the genes involved in disease and defence mechanisms. The results of this study provide a valuable set of data that may help to improve our understanding of the mechanisms employed by plants to tolerate toxic concentrations of metal in the soil.

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.