The process of infection with bacteriophage φX174. XXXI. Abortive Infection at Low Temperatures

Abstract

The infection of Escherichia coli by φX174 at 15 °C is abortive; the cells are killed by the infection but neither mature phage nor SS-DNA are synthesized. Parental RF is formed and subsequently replicated at 15 °C. The RF made at 15 °C shows normal infectivity and competence as a precursor to progeny SS-DNA after an increase in temperature to 37 °C. Some 200 phage equivalents of serum blocking power are synthesized also during 12 hours of infection at 15 °C. The addition of chloramphenicol to 100 μg/ml. to abortively infected cells late in the infection at 15 °C does not block the subsequent maturation of infectious virus following an increase in temperature to 37 °C. The data suggest that, given an adequate pool of the essential proteins and the precursor RF, the concomitant synthesis of protein is not required for the synthesis of SS-DNA or for phage maturation. It is concluded that all of the proteins required for SS-DNA synthesis and phage maturation are present in the abortive infection at 15 °C. Temperature "shift-up" experiments (from 15 up to 41 °C) performed with temperature-sensitive mutants of φX174 and carried out in the presence of chloramphenicol as just described further demonstrate that the proteins coded by φX174 cistrons II, III, IV, V and VII are synthesized at 15 °C and probably adopt their native conformation at this low temperature. The proteins coded by several φX174 strains mutant in cistrons II, III, IV and VII are shown to be functional at 41 °C, when synthesized at 15 °C, although they are inactive when synthesized at 41 °C. These experiments indicate that the majority of ts mutations impose their restriction during the folding of the nascent polypeptide into the native conformation. Three possible causes are suggested for the abortive infection at 15 °C. (a) A virus-coded protein whose role is essential to the infection is made at 15 °C and assumes its native conformation but its rate of activity is too slow at this temperature to sustain the infection process. (b) Virus maturation may involve the formation of a DNA-protein complex and conformational changes which have an energy threshold infrequently reached at 15 °C. (c) A host-coded protein present in uninfected cells, and whose activity is essential to the infection at all temperatures but not to the host at 15 °C, is inactive at 15 °C. An hypothesis of this type is offered which proposes that the temperature-limiting factor in SS-DNA synthesis in vivo may reflect a temperature-dependent property of the host DNA polymerase. The low temperature threshold for SS-DNA synthesis and phage maturation in infected cells is close to 17 °C.

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