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Histidine kinase (HK) NisK is well known to sense lantibiotic nisin for regulating the biosynthesis of nisin. NisK possesses two trans-membrane segments and a large extracellular region and nisin contains 34 amino acids with five lanthionine rings. Unlike most peptide sensing HK with multi trans-membrane segments, NisK is a representative of a group of rarely reported HK that sense peptide as ligand. To reveal how NisK senses nisin molecule to regulate nisin biosynthesis, we constructed a reporter Lactococcus lactis strain with nisRK constitutively expressed and a reporter gene lacZ expressed under the control of promoter P nisA . We showed that the extracellular region of NisK was involved in recognizing nisin. Conserved residues in this group of HK were found in the extracellular region of NisK and mutagenesis of these residues in the reporter strain revealed that several hydrophobic residues including two aromatic residues are crucial for NisK sensing nisin and regulating nisin biosynthesis. Substitutions of hydrophobic regions in NisK extracellular domain showed that the first strand that was rich of hydrophobic amino acids was involved in regulating nisin biosynthesis. A negatively charged residue in the first βstrand also contributed to nisin biosynthesis. Protein binding analyses demonstrated that nisin could not interact with key NisK mutants, indicating these site in the extracellular region of NisK was involved in recognizing nisin.

Bacteria are considered "social" organisms able to communicate with one another using small hormone-like molecules (pheromones) in a process called quorum-sensing (QS). These signaling molecules increase in concentration as a function of bacterial cell density. For most human pathogens, QS is critical for virulence and biofilm formation, and the opportunity to interfere with bacterial QS could provide a sophisticated means for manipulating the composition of pathogenic biofilms, and possibly eradicating the infection. Streptococcus mutans is a well-characterized resident of the dental plaque biofilm, and is the major pathogen of dental caries (cavities). In S. mutans, its CSP QS signaling peptide does not act as a classical QS signal by accumulating passively in proportion to cell density. In fact, particular stresses such as those encountered in the oral cavity, induce the production of the CSP pheromone, suggesting that the pheromone most probably functions as a stress-inducible alarmone by triggering the signaling to the bacterial population to initiate an adaptive response that results in different phenotypic outcomes. This mini-review discusses two different CSP-induced phenotypes, bacterial "suicide" and dormancy, and the underlying mechanisms by which S. mutans utilizes the same QS signaling peptide to regulate two opposite phenotypes.