Bacteria That Stop at the Blue Light

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

This is the second part of a two-part collaboration post with James of Disease Prone. The first part can be found here and covers an introduction to the bacteria Acinetobacter baumannii and its strange behaviour under blue light. This part looks at the biochemical details of the molecule that causes this behaviour.

The ability of A. baumannii to respond to light is dependent on expression of the gene BlsA, which stands for blue-light sensing A. In silico analysis (i.e work done on the computer) showed that this gene is found in several different Acinetobacter species and contains a blue-light-sensing-using-flavin (BLUF) domain at its N-terminal end:

All proteins have two ends, an N-terminal and a C-terminal end. In many bacterial proteins, one end will contain a receptor for a stimulus, while the other end contains an activator which carries out a molecular response to this stimulus. A protein that recognises arsenic for example will have one domain that recognises the presence of arsenic ions, and one domain that activates another protein that causes the cell to respond to the arsenic.

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So far, only a sensor domain has been found in the BlsA protein, this is the BLUF domain. An activator domain is very likely to be present in order for the protein to have an affect, but as yet there are no clues as to what it might be.

As a 'domain' is just a protein loop, and as evolution likes to re-use parts the same domains can often be found in many different proteins. The BLUF domain is no exception. This means that once you have identified a BLUF domain, you can use related proteins with the same domain to gather information about how it works. In every protein it is found in BLUF has the same job - to sense blue light with the help of a small molecule called flavin. In fact, not only is the BLUF domain found in bacteria, it is also used by some eukaryotes; sensing blue light in small algae-like protists (Euglena gracilis, for the protistologists who I know will be interested, third reference below).

Flavin is a popular molecule for controlling blue-light dependent reactions as it can alter the redox state of the protein it is attached to in the presence of blue light. Flavin-associated molecules are found in a variety of plants, as well as in the soil bacteria Rhodobacter which contains BLUF-domain proteins which can be used as transcription factors - proteins which directly affect the expression of the bacterial DNA. In the presence of blue light these proteins can therefore turn on genes to produce a cellular response.

It's not certain whether the BlsA protein of Acinetobacter baumannii functions as a transcription factor but one interesting point is that its activity is temperature dependent. At 24 degrees celcius the BlsA is expressed and the effect is seen, however if the bacteria are incubated at 37 degrees five times less BlsA is produced. The effect of temperature wasn't greatly explored in the paper but it is a very interesting thing to look into as most bacterial experiments are done at only one temperature, the optimal for bacterial growth. While useful for experimental control, this might cause researchers to miss important effects that are physiologically relevant. In the case of the A. baumannii, which is a human pathogen, 37 degrees is probably the signal that it has reached the interior of a human body, so no matter how dark it is, the bacteria will want to stay put!

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