Transcription termination

In contrast to IgM-secreting plasmacytomas, IgH transcription in B cells that express membrane IgM often continues into the Cs exons whether the B cells express both membrane IgM and IgD or only membrane IgM [17]. Thus, the main difference in transcription termination in the p-8 locus occurs on differentiation of a B cell into an IgM-secreting plasma cell, as in the plasma cell there is little transcription read through into the C5 exons. In the case of IgG expression, it has been shown that both in B cell lines that express membrane IgG2a and in myelomas 

The findings that, in B cell lines, RNA polymerase density within the /x-8 locus falls off gradually, rather than at a discrete point and that the density on the Cs gene does not correlate simply with the abundance of 8 mRNA, have also been demonstrated in primary cells [116,119]. Whereas neonatal B lymphocytes contain significantly less 8 mRNA than adult B cells, in both cell types a similar proportion of polymerase molecules continue through CM into Cs [119]. Also consistent with results obtained using cell lines is the observation that B cells treated with lipo-polysaccharide show an altered pattern of transcription termination in that an increased proportion of polymerase molecules now unload between CM and Cs [116]. 

In summary, in antibody-secreting cells that contain little mRNA for the membrane form of the heavy chain, transcription of the heavy chain locus nevertheless continues into and beyond the membrane exons. Similarly, in cells expressing membrane IgM, RNA polymerase molecules often continue transcribing into the 8 locus there does not appear to be a simple correlation between the amount of Cs transcription and the steady-state level of 8 mRNA. 

Multiple mRNAs are produced from a specific immunoglobulin heavy chain lo- 

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These vectors typically have a polylinker adjacent to the SPG promoter. Successive rounds of transcription initiated by SPG RNA polymerase at its promoter lead to the production of multiple RNA copies of any DNA inserted at the polylinker. Before transcription is initiated, the circular expression vector is linearized by a single cleavage at or near the end of the insert so that transcription terminates at a fixed point. 

Rho-dependent transcription termination signals in E coll also appear to have a distinct consensus sequence, as shown in Figure 37—6. The conserved consensus sequence, which is about 40 nucleotide pairs in length, can be seen to contain a hyphenated or interrupted inverted repeat followed by a series of AT base pairs. As transcription proceeds through the hyphenated, inverted repeat, the generated transcript can form the intramolecular hairpin structure, also depicted in Figure 37-6. 

Figure 37-6. The predominant bacterial transcription termination signal contains an inverted, hyphenated repeat (the two boxed areas) followed by a stretch of AT base pairs (top figure). The inverted repeat, when transcribed into RNA, can generate the secondary structure in the RNA transcript shown at the bottom of the figure. Formation of this RNA hairpin causes RNA polymerase to pause and subsequently the p termination factor interacts with the paused polymerase and somehow induces chain termination.

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. 

PolyA Signal This is the polyadenylation signal for attachment of the polyA tail to generate mature mRNA it is important for transcription termination. 

There is a single prokaryotic RNA polymerase that synthesizes all types of RNA in the cell. The core polymerase responsible for making the RNA molecule has the subunit structure Ojpp. A protein factor called sigma (a) is required for the initiation of transcription at a promoter. Sigma factor is released immediately after initiation of transcription. Termination of transcription sometimes requires a protein called rho (p) faaor. This enzyme is inhibited by rifampin. Actinomycin D binds to the DNA preventing transcription. 

RNA polymerase eventually reaches a transcription termination signal, at which point it will stop transcription and release the completed mRNA molecule. There are two kinds of transcription terminators commonly found in prokaryotic genes ... 

NoRC Snf2h Mouse TIP5 Interacts with RNA polymerase I transcription termination factor I (TTF-1), localized to nucleoli [292] and implicated in repression of ribosomal gene transcription [293]. 

Eukaryotic Yeast plasmid or integration into host chromosome by homologous recombination Transient or permanent Amino acid requirement in autotrophic strain heavy metal induction of resistance gene Yeast ori sequence constitutive or inducible promoter transcription terminator... 

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. 

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. 

A hard-to-understand aspect of the "protein-only" theory of prion diseases is the existence of various "strains" of prion proteins. These do not involve differences in amino acid sequence but differences in the conformations of the PrPSc forms and in the glycosylation patterns. dmw How can there be several different conformations of the same protein, all of which seed the conversion of normal PrP into differing insoluble forms In spite of this puzzle, support for the explanation of strain differences comes from a yeast prion system, which involves transcription termination factor eRF3.x z In this system, which involves a prion whose insoluble form can be redissolved by guanidine hydrochloride,aa differing strains have also been described.ybb cc Nevertheless, the presence of the various strains of animal prions, as well as observed vaccination of inbred mice against specific strains,dd may be more readily understood if the disease is transmitted by an unidentified virus rather than by a pure protein.1/U ee/ff In fact, the diseases have not been successfully transmitted by truly virus-free proteins synthesized from recombinant DNA.ee... 

Alternative Sigma Factors Trigger Initiation of Transcription at Different Promoters Elongation of the Transcript Termination of Transcription Comparison of Escherichia coli RNA Polymerase with DNA Poll and PolIII... 

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... 

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