SporeWeb

Bacillus subtilis

Commitment to sporulation and engulfment
(Stages II - III)

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4.Commitment to sporulation and engulfment (Stages II - III)

Commitment to sporulation marks a point of no return and occurs when spatial and temporal activation of sporulation-specific sigma factors is established 57, 83. From this point onwards complex and tightly regulated gene expression, together with refined biochemical communication between different parallel regulatory pathways in the two cell compartments, ensure the formation, maturation and release of the endospore 33, 84

Activation of early stage sporulation sigma factors σF and σE in the forespore and mother cell, respectively, is coupled to the completion of asymmetric cell division. Polar division leads to the confinement of 30% of one of the two chromosomes into the forespore, leaving the remainder plus an additional chromosome in the mother cell 85. This genetic asymmetry, in which the chromosome origin-proximal positioning to the cellular pole plays an important role 70, lays the first basis for compartmentalized gene expression 33, 86. The remainder of the chromosome is then transferred into the forespore by the action of the DNA translocase SpoIIIE 34, 87.

A crucial event in the initiation of compartmentalized gene expression is the activation of σF in the forespore through posttranslational modification. Subsequent activation of σE (and the timing thereof) in the mother cell instigates the expression of the first subsets of sporulation-specific genes, including those known to play an important role in the engulfment process. This partially understood process encompasses the movement and encircling of mother cell membranes around the forespore and the degradation of the separating cell wall material (reviewed in 33), while simultaneous peptidoglycan synthesis is required 88. Two different systems involving the SpoIIQ/SpoIIAH and the SpoIIDMP proteins play important roles in these events 74, 87, 88, 89, 90, 91.

 

The process of engulfment can be subdivided in separate stages marked IIi, IIii and IIiii 90 and includes important checkpoint control systems to ensure that the sporulation process will not be continued unless engulfment progresses properly 92. The location of involved genes on the chromosome (e.g. near the origin of replication) determines proper timing and location of gene expression and protein activation and is crucial for efficient sporulation ( 93, 94).

Synthesis of additional regulatory proteins in both the forespore and the mother cell compartments leads to further moderation of gene expression in a pulsated manner. By either enhancing or repressing the transcription of σF-dependent genes in the forespore or σE- dependent genes in the mother cell, RsfA, SpoIIID and GerR bring about a regulatory circuit of feed-forward loops (FFLs) that are important for further cellular development and provide a tight connection between the early and late stages of sporulation 95, 96. Furthermore, σA continues to be active alongside the sporulation-specific sigma factors throughout the sporulation process, which is required for housekeeping gene expression 97.

 

Sporulation cycle of Bacillus subtilis

 

 

 

Schematic representation of sporulation-specific gene transcription regulation during commitment to sporulation

Completion of an asymmetric septum divides the cell into two unequal parts and lays the basis for compartmentalized gene expression. In the smaller forespore σF is released from the inhibiting function of SpoIIAB by dephosphorylation of SpoIIAA (via SpoIIE). Activation of σF instigates the expression of a subset of genes, including spoIIR, required for the activation of σE in the mothercell. Additional regulators RsfA in the fore spore as well as SpoIIID and GerR in the mother cell help fine tune the expression of sporulation-specific genes. Late stage sporulation-specific sigma factors σG and σK are synthesized but remain inactive (grey) until engulfment is complete. Active proteins are indicated as green ovals. Active sigma factors are indicated as blue hexagons. Their regulons are grouped in red lined boxes (repressed genes) and blue lined boxes (activated genes). Inactive sigma factors are indicated as a grey hexagon (held inactive by additional proteins) or circle (requires post-translational processing before activation). Active and inactive genes important for the sporulation process are indicated as wide black arrows and wide grey arrows, respectively. A positive effect of proteins on gene transcription is represented by black arrowed lines, or blue arrowed lines if the positive action derives from a sigma factor. A negative effect is represented by black stopped lines. Active and inactive promoters upstream of a gene are indicated as green arrows and red arrows, respectively. Phosphate groups are indicated by P.