New photoreceptor sheds light on how cyanobium works

Tokyo, Japan – Scientists at Tokyo Metropolitan University have discovered a new photoreceptor in cyanobacteria whose structure is partially altered, making it sensitive to green/blue-green light. The photoreceptor belongs to a family that normally responds to red/green light in the environment. They identified the parts of its amino acid structure responsible for this behavior. By editing these parts, sensitivity to red and green light could be restored – a remarkable example of molecular “plasticity” in action.

Cyanobacteria, also known as blue-green algae, play a phenomenal role in shaping nature as we know it. They are the first organisms on Earth known to have produced oxygen by using light to break down water molecules. The same chemical process is a distant relative of photosynthesis seen in plants. The chemical apparatuses that support such processes in cyanobacteria are known as cyanobacteriochromes (CBCRs), cyanobacteria-specific photoreceptors; these are amino acid-based constructs that make them sensitive not only to the amount but also the color of light in the environment, helping them adapt to convert energy as efficiently as possible.

But how exactly do cyanobacteria distinguish colors? In the case of CBCRs, they bind a chemical that contains a chemical group known as a linear tetrapyrrole. There is a wide range of linear tetrapyrrole pigments, and their color response depends on the length of the cloud of “conjugated” electrons that run along it. For example, phycocyanobilin (PCB), a blue pigment, is bound by a CBCR and helps it respond to red or green light, reversibly switching between two different absorption states depending on the ratio of red and green light in the environment. On the other hand, CBCR-binding phycoviolobilin (PVB), a pink pigment, responds to violet or yellow light in a similar way. Interestingly, PCB and PVB are a “pair of isomers” because they have the same chemical composition but double bonds at different positions, causing their color to change drastically. The way pigments bind to CBCRs depends remarkably on their specific structure, which is determined by their amino acid sequence. However, the way in which different pigment/CBCR pairs elicit a variety of responses to light is not yet fully understood.

Now, a team of researchers led by Associate Professor Rei Narikawa of Tokyo Metropolitan University has found another piece of the puzzle: They have discovered a new CBCR from the photoreceptor family that should be sensitive to red and green light. Uniquely, this CBCR binds PVB, the pink pigment, and responds to green or blue-green light. Given the differences from normal PVB-binding CBCRs, the team concluded that the pigment was bound to the CBCR in a new, different way. By carefully analyzing the structure, they also identified three important amino acid residues that they believe were key to this unusual response. After modification, the newly edited CBCR was able to coax the PVB back into PCB (the blue pigment), restoring sensitivity to red and green light.

The work highlights the remarkable diversity and “plasticity” of CBCRs, the ability to be shaped or changed, and advances our understanding of how cyanobacteria can “see” the world in color.

This work was supported by JSPS KAKENHI Grant No. JP22H02587425 and TMU Research Fund for Young Scientists of Tokyo Metropolitan University.