Final answer:
In Vibrio fischeri, luxR encodes a transcriptional activator protein while luxI encodes an enzyme for an autoinducer. Quorum sensing is initiated when a sufficient number of bacteria are present, and it uses autoinducers to communicate and regulate gene expression.
Step-by-step explanation:
In Vibrio fischeri, the quorum-sensing genes are luxR, which encodes a transcriptional activator protein, and luxI, which encodes an enzyme that is used to make a molecule called a(n) autoinducer. Quorum sensing is triggered to begin when a sufficient number of bacteria are present. The correct answer to the question is option c) Activator; Autoinducer.
Autoinducers are the signaling molecules used in quorum sensing, secreted by bacteria to communicate with others of the same kind. These can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL), or larger peptide-based molecules, each activating gene expression in specific ways. The AHL binds to transcription factors, initiating gene expression, and as the bacteria population increases, so does the concentration of the autoinducer. This concentration level increases gene expression, including the production of autoinducers, forming a positive feedback loop.