Transcriptional terminator site

Operons that produce the enzymes of amino acid synthesis have a regulatory circuit called attenuation, which uses a transcription termination site (the attenuator) in the mRNA. Formation of the attenuator is modulated by a mechanism that couples transcription and translation while responding to small changes in amino acid concentration. 

With the transcripts of SSVl, the sequence jj-pjjjj TTTTTCT was found immediately upstream of multiple transcription termination sites [124]. It is not clear yet, whether additional elements are required for function. Similar sequences were identified at the 3 termini of other transcription units (e.g., ref. [47]) and also at eucaryal pol 2... 

See Reference No. 10). (a) 1 operon (b) 2 operon. The transcriptional termination sites are arrowed. 

The major mRNA for T4 DNA Pol is apparently transcribed from the —38 position upstream from the first base of the AUG initiator codon (35). The upstream sequence of gene 43 contains the E. colt —10 consensus sequence (TATAAT) at the —45 to —50 positions and a motA promoter sequence (AAGGCTTCG) at positions —66 to —74 (43). Only 13 nt downstream from the UGA termination codon, there is a potential transcription termination site comprising a six-base (GGACCT) inverted repeat. 

It should be noted that the NRE defined in these experiments is distinct from the previously described c-mos UMS sequence (Blair et al., 1984 Wood et al., 1984). The UMS is located approximately 1.4 kb upstream of the c-mos spermatocyte promoter and was identified because it blocked activation of c-mos transforming potential by insertion of retroviral promoters. It is thought to act as a transcriptional terminator, blocking transcription of c-mos initiated at upstream sequences. However, both the spermatocyte and oocyte transcription initiation sites are substantially downstream of the UMS. Moreover, the presence or absence of the UMS does not affect c-mos expression in either microin-jected oocytes (Pal et al., 1991) or transfected NIH 3T3 cells (Zinkel et al., 1992). It thus appears unlikely that the UMS functions as a negative regulator of c-mos transcription from either the spermatocyte or oocyte promoters in somatic cells. 

Most p-independent terminators have two distinguishing features. The first is a region that produces an RNA transcript with self-complementary sequences, permitting the formation of a hairpin structure (see Fig. 8-2la) centered 15 to 20 nucleotides before the projected end of the RNA strand. The second feature is a highly conserved string of three A residues in the template strand that are transcribed into U residues near the 3 end of the hairpin. When a polymerase arrives at a termination site with this structure, it pauses (Fig. 26-7). Formation of the hairpin structure in the RNA disrupts several A=U base pairs in the RNA-DNA hybrid segment and may disrupt important interactions... 

The p-dependent terminators lack the sequence of repeated A residues in the template strand but usually include a CA-rich sequence called a rut (rho rhilization) element. The p protein associates with the RNA at specific binding sites and migrates in the 5 —>3 direction until it reaches the transcription complex that is paused at a termination site. Here it contributes to release of the RNA transcript. The p protein has an ATP-depend-ent RNA-DNA helicase activity that promotes translocation of the protein along the RNA, and ATP is hydrolyzed by p protein during the termination process. The detailed mechanism by which the protein promotes the release of the RNA transcript is not known. 

RNA is removed during processing. Pol II extends the primary transcript well beyond the cleavage and polyadenylation site ("extra RNA") before terminating transcription. Termination signals for Pol II have not yet been defined. 

Figure 28-13 (A) Stereoscopic ribbon drawing of the phyloge-netically conserved 180-residue C-terminal portion of the TATA-binding protein (TBP) from Arabidopsis thaliana. The sequence consists of two direct repeats, giving the protein an approximate twofold symmetry. From Nikolov et al.337 (B) Structure of the corresponding C-terminal core (residues 155-335) of the human TATA-binding protein (TBP) bound to the TATA sequence of a promoter in adenovirus DNA. From Nikolov et al.327 (C) Structure of human transcription factor IIB bound to a TBP from Arabidopsis thaliana, which, in turn, is bound to an adenovirus TATA sequence. Hypothetical B DNA extensions have been modeled at both ends of the DNA segment. The +1 at the left end is the transcription start site and the —43 upstream end is to the right. From Nikolov et al.338 Courtesy of Stephen K. Burley.

Early transcripts. The early left transcript is initiated from the PL promoter. In the absence of N protein this transcript terminates at fL1. In the presence of N protein the polymerase picks up an N protein at the N utilization site, nutL. This makes it possible for the polymerase... 

Note that termination of the transcription reaction takes place more than several hundred base pairs downstream of the poly (A) signal. The termination site is not known in most cases, and there is no well-defined consensus sequence. After the poly (A) signal-mediated cleavage, the 3 part of the transcript is degraded in the nucleus. It is not well understood what contribution, if any, is made by the termination signal to efficient gene expression from an expression vector. 

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