Yeast and bacteria use clever communication systems to coordinate group behaviors. Yeast cells exchange mating factors to find partners, while bacteria use quorum sensing to sync up based on population density.

These single-celled organisms showcase sophisticated signaling pathways. From yeast's MAP kinase cascades to bacteria's two-component systems, they demonstrate how even simple life forms can process complex environmental cues and respond accordingly.

Cell Signaling in Yeast and Bacteria

Yeast mating factor signaling

• Yeast cells secrete peptide pheromones called mating factors
  • a-type cells secrete a-factor
  • α-type cells secrete α-factor
• Mating factors bind to specific G protein-coupled receptors (GPCRs) on the surface of cells of the opposite mating type
  • a-factor binds to Ste3 receptor on α-type cells
  • α-factor binds to Ste2 receptor on a-type cells
• Binding of mating factors to GPCRs initiates a signaling cascade
  • GPCRs activate a heterotrimeric G protein complex
  • G protein complex dissociates into Gα and Gβγ subunits
  • Gβγ subunit activates a mitogen-activated protein (MAP) kinase cascade
    1. Ste20 kinase phosphorylates and activates Ste11 kinase
    2. Ste11 kinase phosphorylates and activates Ste7 kinase
    3. Ste7 kinase phosphorylates and activates Fus3/Kss1 kinases
  • Activated Fus3/Kss1 kinases phosphorylate downstream targets
    • Ste12 transcription factor induces expression of mating-specific genes
    • Far1 promotes cell cycle arrest in G1 phase to synchronize mating partners
• Mating factor signaling leads to cellular responses necessary for mating
  • Polarized growth towards the mating partner forms a mating projection (shmoo)
  • Fusion of a-type and α-type cells forms a diploid zygote (mating)

Bacterial quorum sensing mechanisms

• Quorum sensing enables bacteria to coordinate gene expression based on population density
• Bacteria produce and secrete small signaling molecules called autoinducers
  • Gram-negative bacteria use acyl-homoserine lactones (AHLs) (Vibrio fischeri)
  • Gram-positive bacteria use oligopeptides (Staphylococcus aureus)
• Autoinducers accumulate in the environment as the bacterial population grows
• Bacteria detect autoinducer concentration using specific receptors
  • Gram-negative bacteria use LuxR-type receptors
  • Gram-positive bacteria use two-component signaling systems
• When autoinducer concentration reaches a threshold (quorum), it triggers changes in gene expression
  • Activation of quorum-sensing regulated genes
  • Coordination of group behaviors
    • Bioluminescence (Vibrio fischeri)
    • Virulence factor production (Pseudomonas aeruginosa)
    • Biofilm formation (Staphylococcus aureus)
• Quorum sensing allows bacteria to synchronize activities and act as a multicellular organism
  • Enables bacteria to respond to changes in population density
  • Provides a mechanism for coordinating complex behaviors requiring a critical mass of cells

Yeast vs bacterial cell communication

• Signaling molecules:
  • Yeast mating uses peptide pheromones (a-factor and α-factor)
  • Bacterial quorum sensing uses small molecules (AHLs, oligopeptides)
• Receptors:
  • Yeast mating uses GPCRs (Ste2 and Ste3)
  • Bacterial quorum sensing uses LuxR-type receptors or two-component signaling systems
• Signal transduction:
  • Yeast mating activates a MAP kinase cascade via a G protein complex
  • Bacterial quorum sensing directly activates transcription factors or two-component signaling
• Gene regulation:
  • Yeast mating activates mating-specific genes via Ste12 transcription factor
  • Bacterial quorum sensing activates quorum-sensing regulated genes via LuxR-type or two-component response regulators
• Cellular responses:
  • Yeast mating induces cell cycle arrest, polarized growth, and cell fusion
  • Bacterial quorum sensing coordinates group behaviors (bioluminescence, virulence, biofilm formation)

Signal Transduction in Single-Celled Organisms

• Signal transduction pathways convert external signals into cellular responses
  • Ligands bind to specific receptors, initiating the signaling cascade
  • Second messengers amplify and propagate the signal within the cell
  • Phosphorylation of proteins by kinases is a common mechanism for signal transmission
• Chemotaxis allows single-celled organisms to move towards or away from chemical stimuli
  • Bacteria use two-component signaling systems to detect and respond to chemical gradients
  • Eukaryotic cells like Dictyostelium use G protein-coupled receptors for chemotactic responses