Translational control without proteins

Many bacterial mRNAs contain riboswitches. These are regions of secondary structure upstream of the translated region. They bind to metabolites, and the resulting structural change regulates gene expression, usually by affecting transcription termination or translation initiation. I just learned about these from a new article in Science, which describes two riboswitches that act in tandem to form a NOR gate.

It looks as though all the references for riboswitches are from 2004 onward, so these guys didn't exist back when I was modeling translation. This is yet another translational control mechanism that could lead to the nonlinear relationship between mRNA and protein levels. Note that riboswitches occur in bacteria: this isn't one of those fancy mechanisms that are limited to eukaryotes. Between ribozymes, riboswitches, and RNAi, the role of RNA has expanded greatly in the past 10-15 years since my college textbooks were written.