termination of transcription

MOLECULAR BIOLOGY: ON TRANSCRIPTION TERMINATION: "Transcription termination is important, not least because stopping too late will disrupt the regulation of other genes on the same chromosome; stopping prematurely, meanwhile, would produce truncated and therefore defective mRNAs. . . termination on most genes has been found to occur at various positions rather than at a single site. Moreover, a discrete signal in the DNA that might define the termination site could not be identified. . . Recent work(1-3) address this problem and provides evidence in favor of a striking alternative mechanism: that the transcribing RNA polymerase is "torpedoed". The mRNA is cut while it is still being synthesized, with the liberated region forming the mRNA. The remaining RNA trails out of the transcribing RNA polymerase, and is attacked by an exonuclease -- an enzyme that can degrade the RNA from its free end. This enzyme chases after the polymerase, chewing up the RNA strand as it goes. When it catches up with the polymerase, transcription is terminated."