Commitment to sporulation and engulfment
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.