Trps of the Trade

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When we think of a cell, we might not realize the extent of its intricacy. Sure, cells may be complex, but then there are tissues, organs, organ systems, and entire organisms. Cells are only a really small thing that make up something bigger. A *really* small, and seldom consequential, part of life.

Or so we consciously think.

Underneath that veneer, though, we all know about the existence of viruses, yeasts, and bacteria. Most bacteria are single-celled organisms. (And viruses aren't even cells!) We all know that too, of course; we often hear of them and of their ability to sicken and kill much larger creatures like ourselves. And only then does it start to come into focus... a single cell has to do what a much larger organism can.

A single cell can move around, communicate, absorb nutrients, make decisions, and do everything else it needs to sustain itself. A single cell is farmer, factory worker, doctor, waste collector, and everybody else all wrapped into one. In a sense, a single cell that lives independently, like a bacterium, is more capable than even one of our own human cells.

That capability, the ability to do and to survive regardless of circumstances, is the core of all organisms and the heart of our story.

Cells, you see, are far from a monotonous group. Even disregarding the dissimilarity between bacterium and human, it is evident that electricity-conducting neurons are quite distinct from, say, massively masticating macrophages. And so there is an abundance of variety in all the pathways that keep a cell alive, and that a cell needs to keep itself alive.

This is illustrated very well in the case of tryptophan. Tryptophan? Trip-tuh-fan. It's also abbreviated as Trp.

Tryptophan is an amino acid – a basic building block that all cells need to make proteins. And when I say "need," cells really do need it. When we don't get enough, we become irritable, anxious, and acutely sensitive to pain. Eat too much turkey and blame the excess Trp for making us sleepy? Yes, that too.

Perhaps surprisingly then, we (or rather, our body, since industrial chemists are all so capable now) cannot make tryptophan. We rely on a diverse diet of chicken, peanuts, and milk to supply all the Trp we need. We eat it, but we cannot make it.

Naturally, bacteria do not have that luxury to cannot; their cells need tryptophan too! "Is this a problem though?," you might ask.
Short answer: No, bacteria obviously make tryptophan successfully. Or they wouldn't be around!
Long answer: Yes, yes it is. Just as it takes energy (and time!) to make anything, including to write this article, it takes energy to make tryptophan (time is less a concern on the molecular scale). It also happens that tryptophan is the most energetically expensive amino acid to make. (Coincidence?)

So what do bacteria do? They have multiple options:
1. Don't make any tryptophan.
2. Make tryptophan only when they need it.
3. Always make tryptophan.

And which option best reflects reality? If you guessed 2, that's it. It is wasteful to make too much of something you don't need, and any excess diverts your resources from more important tasks like replicating. (You can think of bacteria as a sort of self-replicating machine.) And even though bacteria don't have neurons, they innately know this too.

At least, bacteria don't drain their energy reserves by churning out tryptophan molecule after tryptophan molecule after tryptophan molecule. Somehow, they know what they need, when they need it, and how exactly to best fill that need. But how?

Molecules are not immutable, well-defined solids. They are always capable of changing. You break a bond here, form another there. You take two molecules and you connect them together. Tryptophan is no different: it's generated through a series of reactions where you start from something common and change enough bonds to get to Trp. And how do you change those bonds? Do they happen undisturbed? No, you have enzymes that help you do that.

This leads us to the "operon." Bacteria regulate genes differently from humans: they string together a whole chain of genes with a related purpose, so they can regulate them all at once. And that's why the Trp operon exists: bacteria can regulate the expression of this one operon by measuring the level of tryptophan.