Dr (α) Jekyl and Mister (β) Hyde: often protein varies.

Dr Jekyll found a way to transform himself in Mr Hide. The ingestion of a serum changes the Dr Jekyll’s physical appearance and his behavior. Mr Hide indulges the Jekyll's vices without fear of detection. Two antagonistic behaviors in one metamorphic human. Here in Cell, RfaH acts in the transcription-translation coupling to regulate gene expression in Escherishia coli. The protein binds to elongating RNA polymerase to activate recruitment of ribosome to nascent mRNA. However one domain in RfaH (called RfaH- CTD) decides whether or not it do it. Indeed, a second protein’s shape is hidden within a unique amino acid sequence. RfaH-CTD undergoes an extreme conformational switch from an all-α to an all-β-fold (from Red to Yellow in the accompanying figure). Remarkably, one version of the RfaH activates while the second inhibits gene expression. Two antagonistic functions in one metamorphic protein.