Light is the most important source of energy for plants for continuous uptake of energy and for themselves to the changes in their natural environment because they cannot move and change their position. So in order to regulate their photosynthetic capacity in view of the changing light environment they have to adapt themselves. For this, plants have evolved a large set of photoreceptors to monitor light quality and quantity, ranging from the UV to the infrared part of the spectrum. Three different photoreceptor classes have been identified and analyzed

  1. UV-B receptors
  2. Blue UV-A photoreceptors, cry1 and cry 2
  3. Red/ far-red reversible phytochromes

Photoreceptors serve to monitor quantity, quality, direction and duration of light and the expression of genes involved in it. The most characterized plant photoreceptors are the phytochromes which are found in higher plants. Phytochromes exist as dimers and have two polypeptides of 125 kDa, each carrying a covalently linked tetrapyrrol chromophore in the N-terminal domain and dimerization domains in the C-terminal domain. This molecule have the capacity for reversible interconversion between the red light-absorbing Pr form and the far-red light absorbing Pfr form following sequential absorption of red and far-red light. Red light absorption changes the conformation of PhyPr to PhyPfr and Pfr form is then transported into the nucleus. In the nucleus it recruits transcription factors such as PIF3 and induces transcription of light-regulated genes, after interacting with the light regulatory cis-acting elements of the target genes. Irradiation with far-red light inactivates phytochrome by changing its conformation back to Pr. Inactivation is followed by disassembling the active transcription complexes and phytochrome is degraded and exported out of the nuclei. The levels of activated phytochrome (Pfr) are fine tuned by dark reversion (Pfr to Pr) and by interactions of other signaling pathways. There is another mechanism, where light absorption is also followed by activation of heterotrimeric GTP-binding proteins. Phytochromes also acts as a typical serine/ threonine kinases that translocates into the nuclei, interact with transcription regulators in a light-dependent fashion and passes signals along phototransduction pathways to modulate Ca2+ and cGMP levels and regulate partitioning of specific transcription factors.

Bacterial Chemotaxis

Chemotaxis is a mechanism by which bacteria efficiently and rapidly respond to changes in the chemical composition of their environment, approaching chemically favourable environments and avoiding unfavourable ones. This behavior is achieved by integrating signals received from the receptors that sense the environment and modulating the direction of flagellar rotation accordingly. Sequential transient phosphorylation of chemotaxis proteins was found to be a key process in signal transduction. The central mechanism of signal transduction involves two families of proteins found in microorganisms and plants. One is histidine protein kinases, which catalyse the transfer of γ-phosphoryl groups from ATP to one of their own histidine residues and the other consists of response regulator proteins, which are activated by the transfer of phosphoryl groups from the kinase phosphohistidines to one of their own aspartic acid residues. The histidine protein kinase that mediates chemotaxis responses is called CheA and the chemotaxis response regulator is known as CheY. The chemotactic response in bacteria is accomplished by signal transmission between two supramolecular complexes- the receptor complexes, located mainly at the pole of the cell and the flagellar-motor complex, randomly distributed around the cell and embedded within the cell membrane. CheY shuttles back and forth between the complexes and transduces the signal from the receptors to the flagella. The receptors transmit a signal that increases CheA autophosphorylation when attractants are absent or repellents are present. Increased CheA phosphorylation leads to an increase in the level of phosphorylated CheY. Phospho-CheY diffuses from CheA freely through the cell and when it encounters a flagellar motor it binds to a flagellar protein called FliM. Phospho-CheY bound to FliM induces tumbling by causing a change in the sense of flagellar rotation from counterclockwise to clockwise. The six to eight flagella scattered over the cell surface rotate co-ordinately to form a bundle during smooth swimming. This bundle is suddenly thrown into disarray when one or several of the motors moves in reverse direction, causing the characteristic tumble that randomizes the direction of the next period of coordinated smooth swimming. The receptor–CheA complex controls the rate of CheY phosphorylation and a phosphatase termed CheZ is responsible for phospho-CheY dephosphorylation.

Quorum Sensing

Quorum signal is a phenomenon where by accumulation of signaling molecules in the surrounding medium enables a signal cell to sense the number of bacteria in the surrounding environment, therefore the population as a whole can make a coordinated response. At critical cell densities binding of regulatory protein to the signal cell leads to switching on of the genes controlled by quorum sensing. Gram-positive and Gram-negative bacteria use quorum sensing communication circuits to regulate a diverse array of physiological activities. Some of the examples of quorum sensing signaling molecules are

  1. N-acylhomoserum lactone
  2. Cyclic Dipeptides
  3. Autoinducer -2 (AI-2)
  4. Small modified peptides
  5. Intraspecies Communication in Gram negative bacteria

The luminescent bacterium Vibro fischeri lives in symbiotic relationship with squid E. scolopes. These bacteria exist as free-living cells or as symbionts in the light-producing organ of an animal host. The host provides a nutrient-rich environment for the bacterium and the bacterium provides light for the host. At low cell densities the bacterial population does not luminance but at high cell density coordinated bioluminescence is observed. Production of light is mediated by AHL molecules synthesized by Lux I protein. When there are few bacteria, cell produces little AHL. As cell density in the surrounding increases signal accumulates. At critical concentration, AHL binds to Lux R and complex binds to Lux box in the DNA sequence and activates expression of Lux CDABE operon resulting in coordinated bioluminescence production. Lux I produced also increases leading to increased synthesis of AHL.