Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 152

A typical factor-independent (or intrinsic) transcription terminator, shown in Figure 2.14, consists of two parts. The first is an inverted repeat. When an inverted-repeat DNA sequence is transcribed into RNA, the RNA can form a hairpin because the two parts of the repeat are complementary to each other. The inverted repeat is followed by a short string of A’s in the template strand, which results in synthesis of a series of U’s in the RNA. Transcription usually terminates somewhere in the string of A’s in the DNA, leaving a string of U’s at the 3′ end of the RNA terminated using this mechanism.

Although the details of the process remain under study, the different elements of the terminator work together to promote termination. The transcription of the U-rich RNA from the A-rich template causes the RNA polymerase to pause, which allows time for the GC-rich hairpin to form in the emerging RNA transcript. The hairpin structure then causes the RNA polymerase to be released by an unknown mechanism, but it is likely to involve hairpin-induced conformational changes and destabilization of the DNA-RNA hybrid, which is facilitated by the fact that the AU base pairs that form in the DNA-RNA hybrid are less stable than a normal DNA-RNA that includes GC base pairs. RNA polymerase releases the RNA transcript and the DNA, terminating transcription, and the released transcript contains the hairpin structure at its 3´ end.