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

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.