SecA IS REQUIRED FOR THREE DISTINCT STAGES OF SPORULATION IN BACILLUS SUBTILIS

抄録

Bacillus subtilis secA341 mutant cells sporulate at 32°C but not at 39°C. Temperature shift-up and -down experiments indicated that the cells were sensitive to the high-temperature treatment from an early stage of sporulation to relatively late stages. Within this temperature-sensitive duration, three discontinuous periods were discriminated by means of pulse-treatment with the nonpermissive temperature. These periods were T_-0.5-T_0.5, T₁-T₂ and T₃-T₅; T₀ indicates the end of exponential growth and subscript numbers represent hourly periods after T₀. When the mutant cells harboring the spo0A-lacZ fusion with the sporulation specific promoter, Ps, were incubated at 39°C from T-_0.5 to T_0.5, the subsequent expression of lacZ gene at 32°C was almost completely abolished. Although the sporulation frequency of these temperature-treated cells was around 1/10 of the control culture, the remaining vegetative cells exhibited the so-called Spo0 phenotypes. Electron microscopic analysis with cells incubated at 39°C after developing to some stages of sporulation at 32°C, suggested that the second temperature-sensitive period, T₁-T₂, corresponded to a SecA-dependent stage for the initiation of asymmetric septation which occurred at around T₃ in the control culture. The third period, T₃-T₅, dependent on the SecA function was shown to correspond to a stage of septum engulfment; the control culture at 32°C completed the engulfment process and forespore formation at T₇.