Transcription Termination at Specific Sites
If transcription of different genes is to be regulated differently, then the genes must be transcriptionally separated. Such transcriptional isola-tion could be achieved without explicit transcriptional barriers between genes if genes were widely spaced and if RNA polymerase occasionally randomly terminated transcription. A more efficient method to separate transcriptional units is merely to have transcription termination signals at their ends.
Figure 5.4 Fusion between two operons to place some of the genes of operonB under control of the promoter of operon A. The fusion removes the transcrip-tion termination signal tA and the promoter pB.
Transcription termination signals can be shown to exist by several types of experiments. One that was first done in bacteria is genetic. As mentioned earlier, transcriptional units are called operons. Even though genes in two different operons may be located close to one another on the chromosome, only by deleting the transcription termi-nation signal at the end of one operon can genes of the second operon be expressed under control of the first promoter (Fig. 5.4).
A second type of demonstration uses in vitro transcription. Radioac-tive RNA is synthesized in vitro from a well-characterized DNA template and separated according to size on polyacrylamide gels. Some templates yield a discrete class of RNA transcripts produced by initiation at a promoter and termination at a site before the end of the DNA molecule. Thus these templates must contain a transcription termination site. Of course, cleavage of an RNA molecule could be mistaken for termination.
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