Some bacteria are able to use a process called photosynthesis to convert energy from sunlight into another form of energy they can use to grow. Within the bacteria, structures known as photosystems are responsible for absorbing light and transferring the energy to other molecules. The levels of light surrounding the bacteria continually fluctuate. To optimize the amount of light they absorb for photosynthesis, the bacteria have receptors that detect light and regulate the activities of the genes that produce photosystems.
One group of bacteria that carry out photosynthesis are collectively known as purple bacteria. These bacteria contain a light receptor called AerR that interacts with a protein called CrtJ, which can directly bind to and alter the activity of genes involved in photosynthesis. AerR senses light by binding to a molecule called vitamin B12, which can absorb blue light, but it was not clear how it affects the CrtJ protein.
Fang, Yamamoto et al. used biochemical and genetic approaches to study AerR in a purple bacterium known as Rhodobacter capsulatus. The experiments show that R. capsulatus makes two different versions of AerR. The larger version only binds to vitamin B12 that is carrying light energy and stimulates CrtJ to activate genes involved in photosynthesis. On the other hand, the shorter version binds to vitamin B12 in the dark and causes CrtJ to repress genes that produce photosystems.
Receptors similar to AerR are found in many bacteria and other single-celled organisms known as Archaea, including in species that do not perform photosynthesis. Therefore, these findings are likely to be useful to researchers studying how bacteria and Archaea sense light in a variety of situations. A next step will be to find out how the different forms of AerR can change the properties of CrtJ.