By manipulating the growth conditions of Escherichia coli both before and after phage MS2 or phage T4 infections, a dependence of phage growth upon postinfection host amino acid biosynthesis can be inferred. Cells grown under repressing conditions for amino acid biosynthesis shifted to amino acid-free medium postinfection (in the absence of further host gene expression) are poor hosts for phage growth, whereas cells grown under derepressing conditions for amino acid biosynthesis are good hosts regardless of postinfection conditions; thus postinfection biosynthesis of amino acids utilizes preformed host enzymes which still function to carry out their biosynthetic pathways during phage infections. Phage MS2 also appears to permit the derepression of host amino acid biosynthetic operons during the infection. A functional dependence of MS2 growth upon postinfection host gene function is perhaps the strongest argument that the RNA phage do not shut off host messenger RNA and protein synthesis.
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