Free introns of tRNAs as complementarity-dependent regulators of gene expression

If there's a lot of tryptophan in the bacteria, the ribosome scoots along the mRNA quickly and a kink forms later in the mRNA that boots the RNA polymerase off of the DNA prematurely, and stops the cell from making the mRNA that encodes for proteins that make tryptophan.

If there is a deficiency of tryptophan in the cell, the ribosome stalls, and a different kink forms in the mRNA that allows the RNA polymerase to keep on transcribing the genes necessary to make tryptophan.

This only works in bacteria, because there's no nucleus, and transcription and translation can occur simultaneously.