SigG Commitment to sporulation and engulfment
SigG during commitment to sporulation
The spoIIIG gene encoding σG is the last gene in a tricistronic operon, preceded by spoIIGA and sigE (spoIIGB) 114. A σA- 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 σG is required 116.
In relation to sporulation, a second promoter directly upstream of the spoIIIG gene is activated by σF that is lateron reinforced by σG. Interestingly, transcription of spoIIIG by σF 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 σG 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 σG activity and the existence of additional inhibitory mechanisms has been suggested 122.