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Fig. 2. Absence of bgs4⁺ promotes lysis in both germinating spores and growing cells. (A) bgs4 spores are able to germinate but lyse before their first cell division. Tetrads from bgs4⁺/bgs4 strain were dissected on YES and YES + 1.2 M sorbitol medium and incubated at 28°C for 1 and 2 days, respectively. Photographs of a tetrad representative for each case, with two bgs4⁺ colonies and two germinated bgs4 spores lysing before cell division, are shown. Arrows indicate the germination site, with cell projections swelling up (+Sorbitol) and finally lysing (–Sorbitol). (B) bgs4⁺ shut-off promotes cell growth arrest after 14 hours of growth on EMM + thiamine medium. Sorbitol only partially protects the bgs4 cells and delays the arrest of cell growth to 17 hours. Log-phase control and bgs4 p81X-bgs4⁺ cells were grown on EMM or EMM + 1.2 M sorbitol liquid media at 32°C, either in the absence (–T, induced) or in the presence (+T, repressed) of thiamine. Cell growth was monitored after 12 or 15 hours of growth with thiamine, depending on the absence or the presence of sorbitol in the medium, respectively. (C) bgs4⁺ shut-off promotes cell lysis and release of cytoplasmic material from either the growing pole or the septum region. Log-phase bgs4 p81X-bgs4⁺ cells grown on EMM + T liquid medium at 32°C for 18 hours were visualized as in Fig. 1C. (D) bgs4⁺ shut-off induces a dramatic decrease in GS activity. Log-phase bgs4 p81X-bgs4⁺ cells grown on EMM ± T as in B were collected at the indicated times and assayed for GS activity as in Fig. 1B. (E) bgs4⁺ shut-off produces actin delocalization and depolymerization prior to cell lysis. bgs4 p81X-bgs4⁺ cells grown on EMM + T as in C were collected at the indicated times, fixed, and stained with rhodamine-conjugated phalloidin. Bar, 5 µm.

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