Let's take a look at how indigo has been made in transgenic tobacco. (The diagram is from this paper).
Indigo will form without any help from indoxyl (which likes to shift back and forth between the two forms as shown) as long as there's oxygen around (which there usually is). If there are glucosyl transferase enzymes (GT) around, indoxyl will turn into stable indican, and you get no dye. Indican can be turned back into indoxyl via another enzyme, glucosidase (GLU).
So, let's say we can make indoxyl in bacteria. We'd suspect to see the following:
- if there's indoxyl (and oxygen) in a cell, they'll make indigo dye and the cells will turn purple/blue,
- if there's indoxyl in the cell and lots of GT enzyme, the indoxyl should turn into indican before indigo dye has a chance to form, and the cells will be white,
- if there's indican in the cell and enough GLU enzyme, the indican should turn back into indoxyl, which will then turn into indigo dye - purple/blue cells again.
What does this have to do with biosensors? Well, if we were to make a cell that produced lots of indican, it would be colorless - but it would have plenty of proto-dye ready to be made. Then, if we could make the GLU enzyme "switch on" in that same cell, we'd hopefully see the cells turn the color of blue jeans, maybe in a matter of minutes.
Before we can even begin doing anything tricky, we have to build cells that can make indoxyl, which left to its own devices will make indigo dye. Here's the good news: our cells are making indigo! This isn't entirely novel, but we're getting a nice, robust color. There are worse ways to start the summer!
Earlier in the week we had a visit from the Nevada Reno iGEM team, and had a mutual spitballing session. We've also been doing independent research and asking around, subjecting our next set of experiments to as much scrutiny as we can muster. The result: we've thought of several different directions we could continue in, a few of which have nothing to do with biosensors.
The deeper you move into a system, the more likely it is you'll see new applications and approaches. The tricky part is figuring out which applications and approaches are most interesting, and most likely to be fruitful.
Terry D. Johnson is a Berkeley bioengineering lecturer and author. He has been co-advising Berkeley's iGEM team since 2008, and will be one of the lead judges for the North America region this year. His writings here do not necessarily reflect the views of iGEM HQ, Berkeley, or possibly himself, after further contemplation.