Phototaxis: Microbial


Phototaxis is light‐regulated movement of motile organisms (microorganisms in the case of microbial phototaxis), usually resulting in their attraction to (positive phototaxis) or avoidance of (negative phototaxis) illuminated regions. Bacteria and archaea often use a time‐biased random walk strategy – choosing directions randomly, but moving for longer time periods in the chosen direction if it happens to be correct. They use various photoreceptors, including xanthopsins, bacteriophytochromes and sensory rhodopsins to sense light, and two‐component histidine kinase‐mediated phosphotransfer systems to regulate movement. Eukaryotic microbes, by contrast, can respond to the direction of light by regulating the direction of movement. Their phototransduction pathways involve protein serine/threonine protein kinases and regulation by second messengers. Representative organisms from each of the three taxonomic domains whose photosensory signal transduction pathways have been studied. Many components of the signal transduction pathways controlling phototaxis have been identified, but much remains to be elucidated.

Key Concepts

  • Phototaxis enables microorganisms to position themselves in an environment that is optimal for survival.
  • Organisms from all three taxonomic domains display phototaxis with differing photosensory signal transduction pathways.
  • Photosensory signal transduction pathways in eubacteria and archaea involve the coupling of signals from photoreceptors to the same signalling pathways as are used by these organisms for chemotaxis.
  • Phototransduction pathways in archaea and bacteria involve two‐component signalling systems that phosphorylate/dephosphorylate proteins at histidine and aspartate residues.
  • Phototransduction pathways in eukaryotic microbes appear to involve protein phosphorylation/dephosphorylation at serine or threonine residues, and the responsible kinases and phosphatases are regulated by second messengers.

Keywords: phototaxis; photomovement; photophobic response; photoreceptor; photosensory transduction; Rhodobacter; Synechocystis; Halobacterium; Chlamydomonas; Dictyostelium

Figure 1. A model for photosensory and chemosensory signalling pathways in Rhodobacter sphaeroides. Photoactive yellow protein (PYP) elicits repellent responses via an as yet unknown pathway (indicated by the question mark) that probably involves a CheA‐like histidine kinase and a CheY‐like response regulator. Attractant responses to light are elicited by increases in the rate of photosynthetic electron transport (blue arrows), which is coupled to proton pumping (brown arrow) across the photosynthetic membranes. The rate of electron transport may be sensed via the FAD prosthetic group of an Aer‐related methyl‐accepting chemotaxis protein (MCPAer), which, in turn, passes attractant signals to CheWII, CheAII and thence to a CheY‐related protein. Chemotactic attractant signals are passed via multiple CheW, ‐A and ‐Y proteins to converge with the signalling pathways for phototaxis. Genes involved in chemosensory and photosensory signal transduction are found in multiple loci, as shown in the inset. All are transcribed from left to right and some are transcribed as parts of large polycistronic transcripts from the operons shown. Red arrows indicate repellent signals, and green arrows indicate attractant signals. Attractant signals inhibit, whereas repellent signals enhance, the rate of phosphorylation of CheY. Interaction of the phosphorylated form of CheY with the flagellar motor switch causes the motor in Rba. sphaeroides to cease rotation.
Figure 2. Model for control of positive phototaxis in Synechocystis. Directional illumination of the cell produces a sharply focused and intense spot of light (resembling a photonic nanojet) at the cell periphery on the opposite side from the light source. The focused spot is perceived by photoreceptors in the cytoplasmic membrane (for example PixJ1) triggering signal transduction via CheY‐like response regulators that locally inactivates the T4P motility apparatus, dispersing T4P components including the extension motor PilB1. Consequently, patches of the motor proteins can only form on the side of the cell facing the light source. Pili are extended and retracted at this side of the cell, which, therefore, moves towards the light. T4P, Type IV pili. Schuergers et al. . CC BY 4.0.
Figure 3. A model for photosensory signalling pathways in Halobacterium salinarum. Interconversions between the different forms of the two photoreceptors SRI and SRII are indicated by black arrows, and the yellow hv labels indicate which steps are light‐driven. SRI can exist in both attractant (S373) and repellent (Sb510) signalling states and S373 is converted back to SRI587, either via Sb510 in a fast light‐dependent pathway or via a slower light‐independent route. The signalling state(s) of SRII pass repellent signals to the flagellar motors. Attractant signals (green arrows) inhibit, and repellent signals (red arrows) enhance, the rate of phosphorylation of CheY. Interaction of the phosphorylated form of CheY with the flagella motor switch is dependent on the adaptor protein CheF and causes reversal of the direction of motor rotation.
Figure 4. A model for Dictyostelium slug photosensory and thermosensory signalling pathways. The top panel shows a pseudocolour image of two slugs migrating on a water agar surface towards a light source to the right of the figure. Each slug is about a millimetre in length and migrates across the agar surface at about 1 mm h−1. The lower panel shows a lateral view of a migrating slug. The ‘thought bubbles’ emanating from the tip indicate that the tip controls slug behaviour via the indicated pathways. Signals from photoreceptor and thermoreceptor converge early and thence control the concentrations of the intracellular second messengers cAMP, cGMP, Ca2+ and possibly IP3. The evidence for inositol polyphosphate (IP3) involvement is the pharmacological effect of Li+, whose target could also (or instead) be glycogen synthase kinase 3 (GSK3). Heterotrimeric (Gα1βγ, Gα4βγ, Gα7βγ and Gα8βγ) and small (RasD) GTP‐binding proteins and their regulators (MEGAP1 or WAVE‐associated RacGAP Protein (WRP), RasGEFE and RasGEFL) are involved in transducing the signals to regulate components of the actomyosin cytoskeleton. At least some of the signalling proteins (RasD, Erk2, PKB and GRP125) are recruited by the actin‐binding scaffolding protein filamin into a photosensory signalling complex. This modulates the tip activation and inhibition signals that determine the position of the slug tip. Transient lateral imbalances between tip activation and inhibition thus induced by light and temperature gradients cause temporary lateral shifts in tip position and thence slug turning because the slug ‘follows its nose’. Depending upon whether tip activation or inhibition dominates the response, the slug turns either towards or away from the light source, or up or down the temperature gradient. Sign reversals in slug turning responses result from switches in the balance between control by tip activation and inhibition. This explains direction‐dependent sign reversals in phototaxis that cause bidirectional phototaxis and temperature‐dependent sign reversals in thermotaxis that cause movement towards the warmth or the cold depending on the temperature. Tip activation signals are believed to be carried by three‐dimensional spiral scroll waves of extracellular cAMP (shown in cyan in the lower panel, with the direction of propagation in red) analogous to the two‐dimensional cAMP waves that mediate aggregation. At the rear boundary of the tip region, the wave twists and breaks into a train of planar waves that propagate backwards through the rear of the slug. Candidate tip inhibitors are slug turning factor (STF), ammonia and adenosine. Some of the elements used in this figure were kindly provided by Dr Siegert F, Ludwig Maximilian University, Munich, Germany.


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Further Reading

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Storey, Claire, Allan, Claire Y, and Fisher, Paul R(Mar 2020) Phototaxis: Microbial. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000399.pub3]