Cell-Cell Signaling and Quorum Responses

Many organisms use cell-cell signaling to monitor population density, crowding, and colonization to regulate a range of complex processes, including development, pathogenesis, growth, mating, and transformation. In microbes, this is typically done with secreted signaling molecules and dedicated response pathways. The ability to sense and respond to high population density is a type of cell-cell signaling often referred to as quorum sensing. We have characterized several types of regulatory responses that are controlled by secreted peptides in B. subtilis. Some of these responses are conserved and used generally by gram positive organisms to modulate gene expression, development, pathogenesis, and horizontal gene transfer.

In B. subtilis, one of the major pathways for quorum sensing involves activation of the transcription factor ComA, a response regulator that is active when phosphorylated. The activity of ComA is modulated by at least two different types of peptide signaling molecules that accumulate in culture supernatants as cells grow to high population density. We used DNA microarrays to identify genes controlled by ComA and the peptide signaling pathways, and have characterized the DNA binding site for ComA. We are interested in the regulatory networks affected by combinations of transcription factors, including ComA.

We identified several secreted peptides that influence gene expression and development in B. subtilis. One type of peptide functions outside the cell by interacting with the receptor kinase (ComP) thereby stimulating the activation of the transcription factor ComA. This mechanism has been found to be generally conserved in gram positive bacteria. We found that in B. subtilis, this pathway stimulates the development of genetic competence (the ability to bind and take up exogenous DNA), and stimulates expression of many genes involved in a general response to high population density (quorum sensing).

The second type of peptide signaling pathway that we have characterized involves a family of pentapeptides that are exported, accumulate outside the cell, and then are imported through an oligopeptide permease. Once inside the cell, these peptides interact with a cytoplasmic receptor (called Rap). The receptor normally inhibits the activity of a target transcription factor (often indirectly). When bound to ligand, the Rap protein is antagonized, typically allowing the target transcription factor to be active. We found that one of the pentapeptides stimulates a general quorum response (including the development of genetic competence) and also stimulates sporulation. Another peptide of this type inhibits activation of the integrative and conjugative element ICEBs1.