Field of Science

Chameleon plants!

ResearchBlogging.orgThis 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 (both freshwater, marine and soil varieties) have the ability to change their colour depending out external conditions. The reason 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) two proteins capable of absorbing energy from light: Phycocyanin (PC) and Phycoerythrin (PE).

These two proteins both absorb light at different wavelengths, and the light wavelengths that they don't absorb get reflected away from the bacteria and (if you're standing by) into your eyes where after complex eye/brain interactions, 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, imaginatively called RcaE, RcaF and RcaC (other letters were found but have either not been properly characterised, or turned out to be less important). Unsurprisingly, given that this is a bacterial system, it's mostly a modified two-component response. RcaF is the sensor, with a chromophore binding domain that holds a light-sensor, and a terminal kinase domain that passes the signal on the the receivers.

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 (absorbing red light and turning the cells green). As usual with two-component systems, the signal is passed on by phosphorylation.

Under green light everything gets dephosphorylated and the PC production is turned off. This dephosphorylation causes RcaC to bind to the promoter for the genes that produce PE (absorbing green light and turning the cells red). Green light also activates another system called the Cgi system (which stands for Control of Green light Induction). This turns on PE production, although it hasn't been properly characterised as yet, so the actual biochemical mechanism for this is unknown.

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...


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


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SimonG said...

Bacteria really do some clever stuff.
I wonder if this is more efficient than having a single pigment which absorbs more wavelengths? It seems like it ought to be.

Lab Rat said...

Thanks for the comment! I get the feeling that pigments are better at absorbing light energy when they do it over a narrower range (although I'd have to check with a physicist). And seeing as bacteria *have* different pigments with different wavelength abilities, this is quite an efficient way of using them.

(in some marine bacteria this is also hooked up to the gas vesicle system, so that as the bacteria float up towards the surface of the sea they can start preparing for the change in light)

cuco3 said...

I'd think that the pigments themselves would be more efficient, but then you've got to have two lots of mechanisms to produce them. So they make bespoke pigments rather than taking a generic one off the shelf.

Lab Rat said...

My guess is that the bacteria would have started off with one light-capturing pigment. A gene copying event would then lead to two copies, one of which would specialise to a different wavelength, allowing the cell to choose between them.

Either that or the pigment evolved to fit two different wavelengths in different bacteria, and then just got passed between them, until they all had both. There are quite a lot of different wavelength capturing pigments out there by the time you include all the ones in plants and odd algae.

(this is all speculation by the way, unfortunately I don't have the time to dig into it at the moment, but I will ask my lecturer when I next see him!)