The RRNPP family
In addition to be sensed by the receptor histidine kinase of two component system, the peptide can be actively imported back into bacteria by the oligopeptide transporter Opp, at which point they interact with sensors in the cytoplasm. These sensors belong to the RRNPP family of cytoplasmic regulatory receptors. The RRNPP family is named for the prototypical members, Rap, Rgg, NprR, PlcR, and PrgX. The RRNPP members that have been studied are found in bacilli, streptococci, or enterococci. The C-terminal regions of the RRNPP proteins adopt a tetratricopeptide repeat (TPR) domain-like conformation which binds the signaling peptides. The signaling peptides are linear and exhibit mature lengths between 5 and 10 amino acids. With roles in virulence, natural competence, sporulation, biofilm formation, and other activities, RRNPP signaling pathways stand as attractive targets for treatments aimed at manipulating bacterial behaviors.
Figure 1. Schematic representation of the RRNPP mechanism involving transcriptional regulators, cited from reference [1].
The AimR-AimP system of Bacillus subtilis bactriophage phi3T
The bacteriophage can employ the RRNPP family proteins to make the lysis-lysogeny decision. In a paper published on Nature in 2017, the authors found the B. subtilis bacteriophage phi3T encode the Aim system to make lysis-lysogeny decision. The AimR is a transcription factor, which has a helix-turn-helix domain to bind DNA and a TPR domain to bind the signal peptide. The AimP is the propeptide of the mature signal peptide, sequence of the mature signal peptide is SAIRGA. Binding of AimP to the AimR will disrupt the dimer forms of AimR. After that, the AimR can no longer bind to the promoter of AimX, a potential non coding RNA involved in the process of lysis-lysogeny. A superb characteristic of the Aim system is that the AimR only bind to the promoter of the AimX gene. The ChIP-seq data revealed that there is only one binding site of the AimR in the whole hpi3T genome. This data indicate that the specificity of the Aim system is very high, which is the required characteristic for the synthetic QS system.
Figure 2. The AimR-AimP system and its role in the phage lysis-lysogeny decision, cited from reference [2].
The PlcR-PapR system of B.cereus
B. cereus cause acute diarrheal disease by the production and secretion of a variety of hemolysins, phospholipases, and toxins. The production of virulence factors is controlled by the PlcR-PapR QS system. PapR is 48 amino acids long and contains an N-terminal signal peptide that targets it for secretion. Outside the cell, the PapR pro-AIP is processed by the secreted neutral protease B (NprB) to form the active AIP. The mature PapR oligopeptide sequence is ADVPFEL. The processed PapR AIP is imported back into the cell by the oligopeptide permease system (Opp). The PlcR protein has two domains. The helix-turn-helix domain is involved in the DNA binding activity. The TPR domain is involved in signal peptide binding. Inside the cell, PapR binds to the transcription factor PlcR, and this causes conformational changes in the DNA-binding domain of PlcR, facilitates PlcR oligomerization, DNA binding, and regulation of transcription. The plcA gene is under the control of PlcR, its promoter is usually used to construct reporter system.
Figure 3. PlcR activation Model, cited from reference [3].
Introduction of B. subtilis
As a model organism, B. subtilis is commonly used in laboratory studies directed at discovering the fundamental properties and characteristics of Gram-positive spore-forming bacteria. Due to its excellent fermentation properties, with high product yields (20 to 25 gram per litre) it is used to produce various enzymes, such as amylase and proteases. Other enzymes produced by B. subtilis are widely used as additives in laundry detergents. It is also used to produce hyaluronic acid, which is used in the joint-care sector in healthcare and cosmetics. B. subtilis is the most widely used Gram positive bacteria chassis in synthetic biology. It has its own QS systems. However, to avoid interfering with its own physiology, synthetic communication system is needed for this important chassis.
References:[1] Perez-Pascual, D., Monnet, V., and Gardan, R. (2016). Bacterial Cell-Cell Communication in the Host via RRNPP Peptide-Binding Regulators. Front Microbiol 7, 706.
[2] Erez, Z., Steinberger-Levy, I., Shamir, M., Doron, S., Stokar-Avihail, A., Peleg, Y., Melamed, S., Leavitt, A., Savidor, A., Albeck, S., et al. (2017). Communication between viruses guides lysis-lysogeny decisions. Nature 541, 488-493.
[3] Grenha, R., Slamti, L., Nicaise, M., Refes, Y., Lereclus, D., and Nessler, S. (2013). Structural basis for the activation mechanism of the PlcR virulence regulator by the quorum-sensing signal peptide PapR. Proc Natl Acad Sci U S A 110, 1047-1052.