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SigG Commitment to sporulation and engulfment (Stages II - III)

SigG during commitment to sporulation

The spoIIIG gene encoding σ^G is the last gene in a tricistronic operon, preceded by spoIIGA and sigE (spoIIGB) ¹¹⁴. A σ^A- and Spo0A-dependent promoter in front of this operon is responsible for early transcript of spoIIIG, which is not translated into protein due to blocking of the ribosome binding site ¹¹⁵. Chary et al. ¹¹⁶ point out that the chromosomal location of the spoIIIG gene immediately downstream the spoIIG locus is conserved in spore forming bacteria, even though the read-through transcript from the spoIIG promoter was found to be non-essential for spore formation ¹¹⁷. The authors postulate that the block in mRNA translation can be overcome in a so far unknown specific situation in which some production of σ^G is required ¹¹⁶.

In relation to sporulation, a second promoter directly upstream of the spoIIIG gene is activated by σ^F that is lateron reinforced by σ^G. Interestingly, transcription of spoIIIG by σ^F is delayed by approximately one hour upon near completion of the engulfment process and depends on a yet unknown signal from the mother cell ^{100, 118}. The transcript is then translated into a functional σ^G protein that is kept inactive until the engulfment process is complete through direct binding by CsfB, also referred to as Gin ^{119, 120}. However, the presence of CsfB is dispensable for blocking σ^G activity and the existence of additional inhibitory mechanisms has been suggested ¹²².