Photoactive yellow protein

Photoactive Yellow Protein (PYP) is a small cytosolic photoreceptor thought to be responsible for the negative phototactic response of its host organism. It consists of a single polypeptide chain of 125 residues and the chromophore para-hydroxy cinnamic acid, which is covalently bound to Cys69, the only cysteine residue in the protein.

The photocycle

In the dark, the PYP chromophore is in the trans, deprotonated state and the protein is well folded. This state absorbs in the blue (so it looks yellow). When a blue photon is absorbed, the chromophore can isomerize to the cis state and become protonated. The cis state absorbs near 350 nm so appears colourless. In the cis, protonated state the protein partially unfolds. When the blue light source is removed the protein spontaneously refolds and the chromophore reisomerizes to the trans form.

Photocontrolled transcription factors

Our aim is to use PYP as a genetically encoded light switch that can be fused to transcription factors like GCN4 and control their activity. The idea is to use the photo-controlled folding/unfolding process of PYP to sterically prevent/allow an interaction required for function. For more information see: J Mol Biol. 2010;399(1):94-112.


The idea of using genetically encoded light switches to control biochemistry in living organisms has been termed "optogenetics". The best developed optogenetic tools are the channelrhodospins which are already leading to new insights into how complex things like living brains function. The potenial of such tools is highlighted in the 2010 Method of the Year section in Nature Methods.