INHIBITORS FOR STAGE 2
"The inhibitors include at least three Rap proteins [Spo0F-P phosphatases], Spo0E [Spo0A-P phosphatase], KipI and SinR. Each has a specific antagonist."
Rap proteins
We currently (as of 2000) know of 3 Rap proteins.
"For instance, RapA is inactivated by a pentapeptide encoded by phrA."
ALL Rap proteins are inhibited by specific pentapeptides.
"Recent work has identified the third known phosphatase for Spo0F~P (RapE) and its antagonist (PhrE)"
How the pentapeptides are regulated is currently unknown - it's also not known why 3 are needed.
"How the rate of intracellular accumulation of phosphatase-inhibiting pentapeptides is regulated is also unclear; it may simply be a passive process, reflecting the time required for synthesis, secretion and processing of precursors of about 40 amino acids, and subsequent uptake of the pentapeptides. Alternatively, the cell may depend on further nutritional or physiological cues to set the time at which a sufficient level of Spo0F~P accumulates to allow commitment to sporulation."
KipI
"KipI, an inhibitor of KinA,
How is it repressed/inhibited?
"...is antagonized by KipA"
SinR
"SinR, a tetrameric protein, is synthesized constitutively and binds to the promoter sites of the spo0A and spoIIG genes, repressing their transcription..."
"Relief of repression is mediated by SinI, a protein that is induced as cells enter stationary phase"
How is it repressed/inhibited?
"SinI forms a 1:1 complex with SinR monomers, driving SinR from its active, tetrameric state"
"The interaction between the two proteins is mediated by nearly identical α-helical domains"
Soj
<fill in>
ScoC
"ScoC ... has at least three important targets: the promoters of the opp and app oligopeptide transport operons and the sinI gene."
"The oligopeptide permeases are needed to import the signaling pentapeptides that inactivate Rap phosphatases and SinI is needed to antagonize SinR (see above). Thus, repression by ScoC must be relieved, by an unknown mechanism, for sporulation to ensue."
Structure of the Spo0A phosphorelay proteins
<fill in>
The phosphorelay system was first discussed in detail in \cite{Hoch_1993}. The following quotes are from the linked article (emphasis mine).
"The major signal-transduction pathway for the initiation of sporulation is the phosphorelay, which responds to environmental, cell cycle, and metabolic signals, and phosphorylates the SpoOA transcription factor activating its function."
"in a normal culture grown in the laboratory, sporulation occurs during the stationary phase, and many of the processes, alternate pathways, and enzymes formed during the early part of stationary phase are controlled along with sporulation because the cell controls sporulation and many of the stationary-phase processes by a single transcription factor, SpoOA."
There are two main external factors that can trigger sporulation: nutrient starvation and bacterial population density.
Nutrient starvation can be detected inside the cell by utilising GTP/GDP concentrations. In high concentrations, GTP binds to the protein CodY. CodY-GTP acts as a repressor to many early sporulation genes.
Population density can be detected through quorum sensing.
If either one of these external conditions is detected, the cell initiates a phosphorylation cascade from the histidine kinase, KinA through two intermediate proteins, Spo0F & Spo0B, to the transcription factor Spo0A, forming Spo0A-P. Spo0A-P represses the transcription of AbrB, which is a repressor to many genes responsible survival strategies such as swimming to a new site, catabolizing macromolecules and killing competitors. AbrB is also a repressor to KinA transcription, hence repression of AbrB allows the potential increase of Spo0A-P concentrations if external conditions do not improve. In high concentrations Spo0A-P acts as a positive regulator for many sporulation genes such as spoIIA, spoIIGA and spoIIE, triggering the second stage of sporulation.
Thank you X. As X mentioned, I'm going to be talking a little bit about how sporulation is triggered, but more importantly I'm going to discuss some of the ways the cell tries to avoid sporulation.
Cells capable of sporulation are in a balancing act under poor conditions, because sporulation is a metabolically expensive process, that takes up to 8 hours to complete, which is a long time when you have a generation time of 20 minutes! It has to commit itself to the process but it can be too costly to come back from if its wrong. The cell does not want to form an endospore unless it is certain it is the right course of action, however if sporulation in this environment is inevitable, then the cell wants to make sure it sporulates first, before the bacteria surrounding it have, <for reasons we'll discuss later>. For now I'm going to focus on three main questions: How does the cell detect that the outside conditions have become so unfavourable? How does the cell make sure this isn't a false positive (because if the poor conditions are temporary the cell has made a costly mistake)? And providing it isn't, how does it begin the process of forming an endospore?
So let's look at the first question, how does the cell detect the unfavourable outside conditions? Well the first way is actually something we've covered, and that's quorum sensing. As you already know how this works, I'll skip over the details and tell you that detecting high concentrations outside the cell triggers a phosphorylation cascase, which looks like this. Our histidine kinase, KinA becomes phosphorylated, and passes this down the chain