Models for the evolution of self-regulation by RNA- and DNA-binding proteins. (a) RNA-binding protein. Among other cellular functions involving interactions with RNA molecules, RNA-binding proteins often regulate their own expression by interacting with the 5' UTR of their mRNA. In the case of transcriptional attenuation, the transcription start site in the DNA is followed by a palindrome corresponding to the terminator. Emergence of an additional transcription start site downstream of the terminator palindrome could give birth to a new transcription factor binding site. (b) DNA-binding protein. These proteins (mainly transcriptional factors) often regulate their own expression through interaction with a palindromic DNA sequence in the promoter. Emergence of an additional transcription start site upstream of this transcription factor binding site palindrome could give birth to a new transcriptional attenuator.