Mobile elements and the emergence of new attenuators. (a) 5' terminators detected upstream of transposase or insertion sequence (IS)-related genes correspond to intrinsic IS 5' elements that prevent expression of the IS when it is inserted in a transcribed region. (b) ISs also contain intrinsic 3' terminators that terminate transcription of the 3'-most IS genes. When IS-related genes undergo pseudogenization, some 3' terminators may remain active as attenuators of downstream cellular genes (exaptation). As IS insertion may occur at nearly random genomic sites, similar attenuator candidates may be found upstream of totally unrelated genes.