In the study of bacteria, a longstanding dogma has held that two molecular machines—RNA polymerase, which leads the way in transcribing DNA into RNA, and ribosomes, which bring up the rear translating RNA into proteins—worked so closely in tandem that they were effectively attached. This close coupling of transcription and translation in bacteria was thought to be fundamental to gene expression, in part because the trailing ribosome could shield nascent gene products from an effective and omnipresent quality-control protein called Rho.