SECA IS REQUIRED FOR 3 DISTINCT STAGES OF SPORULATION IN BACILLUS-SUBTILIS

Bacillus subtilis secA341 mutant cells sporulate at 32 degrees C but not at 39 degrees 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, T-1-T-2 and T-3-T-5; T-0 indicates the end of exponential growth and subscript numbers represent hourly periods after T-0. When the mutant cells harboring the spoOA-lacZ fusion with the sporulation specific promoter, Ps, were incubated at 39 degrees C from T--0.5, to T-0.5, the subsequent expression of lacZ gene at 32 degrees 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 SpoO phenotypes. Electron microscopic analysis with cells incubated at 39 degrees C after developing to some stages of sporulation at 32 degrees C, suggested that the second temperature-sensitive period, T-1-T-2, corresponded to a.SecA-dependent stage for the initiation of asymmetric septation which occurred at around T, in the control culture. The third period, T-3-T-5, dependent on the SecA function was shown to correspond to a stage of septum engulfment; the control culture at 32 degrees C completed the engulfment process and forespore formation at T-7.