OSMOTIC STRENGTH BLOCKS SPORULATION AT STAGE-II BY IMPEDING ACTIVATION OF EARLY SIGMA-FACTORS IN BACILLUS-SUBTILIS

In Bacillus subtilis, osmotolerance is a stationary phase-dependent, a daptive response inhibiting sporulation and sharing common regulators with this process. The extent of this inhibition was determined by mea suring transcription activity of promoter lacZ fusions to early sigma genes (spoIIG and spoIIA coding for precursors of sigma(E) and sigma(F )) and to reporters of them (spoIID, spoIIQ and spoIIIG), in the absen ce and presence of 0.6 M or 1 M NaCl. The transcription activity of th ese sigma precursors, normally occurring at the onset of the stationar y phase, was reduced to 30-50% of their maximal expression in hyperosm otic conditions; expression of genes under their control was, however, more inhibited (< 10%). Therefore, sporulation was blocked at the sig ma sigma(E) and sigma(F) activation steps. This assumption was confirm ed by electron microscopic examinations of hyperosmotic cultures, whic h presented asymmetric septa characteristic of stage II mutants. Discu ssion was focused on the particular composition and/or structure of me mbranes during hyperosmotic growth and their involvement in the arrest of sporulation.