This post was originally published in "Life of a Lab Rat" on Wednesday 3rd February 2010.

Chameleon bacteria

This is a picture of a small cyanobacteria under red light:

And this is a picture of exactly the same organism under blue-green light:

Some cyanobacteria have the ability to change their colour depending on external conditions. They do this is because they photosynthesise and therefore require light for energy. The light is harvested by the bacteria using special protein complexes called phycobilisomes which contain (among other things) proteins capable of absorbing energy from light. The colour-changing cyanobacteria have two main varieties: Phycocyanin (PC) and Phycoerythrin (PE).

These two proteins both absorb light at different wavelengths, and the wavelengths that they don't absorb get reflected away from the bacteria and into your eyes where you perceive them as colour. This is why plants look green, because chlorophyll in the leaves absorbs all the red and blue light. The two light absorbing complexes in the phycobilisome both absorb at different wavelengths as shown in the absorbence spectrum below:

The colour bar below shows what wavelengths of light are being absorbed by the PE and the PC. PE absorbs green light and therefore looks red, while PC absorbs red light and therefore looks green.

One of the main systems involved in controlling this colour change is called the Rca system (stands for Regulator of Complementary Chromatic Adaptation). This consists of three main proteins: RcaE, RcaF and RcaC (other letters were found but have either not been properly characterised, or turned out to be less important). RcaF acts as a sensor, with a chromophore binding domain that holds a light-sensor, and a terminal kinase domain that passes the signal on.

RcaF and C are both response regulators, although it's proposed that RcaF is an intermediate responder, which passes the message on to RcaC. RcaC contains a DNA binding domain, which allows it to activate the genes required to turn on genes for PC production. The PC absorbs red light, and the cells turn green.

Conversely, under green light everything gets dephosphorylated and PC production is switched off. The dephosphorylation also causes RcaC to bind to the promoter for the genes that produce PE, which absorbs green light and turns the cells red. This allows the cells to toggle between red and green depending on what wavelength of light they wish to absorb.

What the paper doesn't say, and what I really want to know, is whether anyone's tried to make tartan patterned bacterial populations with this...


Reference:Kehoe, D., & Gutu, A. (2006). RESPONDING TO COLOR: The Regulation of Complementary Chromatic Adaptation Annual Review of Plant Biology, 57 (1), 127-150 DOI: 10.1146/annurev.arplant.57.032905.105215 -

All images are taken from the reference