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J Virol. 1976 January; 17(1): 94-105
Copyright © 1976 American Society for Microbiology. All Rights Reserved.

T7 Protein Synthesis in F-Factor-Containing Cells: Evidence for an Episomally Induced Impairment of Translation and Its Relation to an Alteration in Membrane Permeability

¹ Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111

ABSTRACT

T7 infection of F-factor-containing PIFA⁺,B⁺ cells is abortive. In spite of the presence of mRNA for all three classes of T7 proteins, only the earliest of the T7 proteins are synthesized. A crucial question is whether the failure of T7 to develop in PIFA⁺,B⁺ cells is the result of an inability to translate the late classes of T7 mRNA or, as has been recently suggested (Britton, and Haselkorn, 1975; Condit, 1975), whether it is the result of a more generalized alteration in membrane permeability. We have examined the effects of the wild-type PIFA⁺,B⁺ episome and two episomal mutations (pifA^– and pifB^–) on in vitro translation and membrane permeability. In vivo the episomal mutations allow partial or complete T7 development to occur. We demonstrate that cell-free protein-synthesizing systems from T7-infected PIFA⁺,B⁺ cells show a three- to fivefold decrease in the rate of translation of both natural and synthetic mRNA. In addition, ribosomes from T7-infected PIFA⁺,B⁺ cells are defective in their ability to bind Fmet tRNA_f in response to natural mRNA. By contrast, cell-free extracts from T7-infected pifA^– (PIFA^–,B⁺) cells retain the ability to bind Fmet tRNA_f and to translate natural and synthetic mRNA at normal rates. The defective T7-infected PIFA⁺,B⁺ ribosomes can be restored to full activity by a trypsin-sensitive fraction from uninfected PIFA⁺,B⁺ or T7-infected PIFA^–,B⁺ cells. Despite the differences in translational capacity of these extracts, both T7-infected PIFA⁺,B⁺ and PIFA^–,B⁺ cells display the same permeability lesions as measured by the loss of ATP from the cells into the supernatant. Mutation of the episome to pfiB^– prevents the loss of ATP from the cells after T7 infection.