Termination

It is built new for every cut: five short RNAs, a hundred-odd proteins gathering on one intron, taking it out, joining the two ends, and the message leaves.

Then the part nobody had pictured. The machine still holds what it removed, a loop knotted at one letter, wedged where the cutting was. A dead end, the paper calls it. It is.

So one helicase pulls the knot one way, another pulls a strand the other, an enzyme hung on by its tail snips the loop into a line, and the last one walks the RNA and lets the line go.

In Vienna they made that last one unable to burn its fuel. It sat down on the machine like the real one, fitted, held on, did nothing wrong, and nothing came apart.

It knew how to hold. That was the whole of what it knew.