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Mouse erythroleukemia (MEL) cells provide a useful model system to examine the regulation of globin gene expression. MEL cells ordinarily do not express globin genes, but in the presence of inducers, such as dimethyl sulfoxide or hexamethylene bisacetamide, they mimic erythroid differentiation. We have developed a cell-free transcription system from uninduced MEL cells to determine the requirements for mRNA synthesis. The MEL system directs accurate transcription of adenovirus type 2 major late DNA and mouse betamaj-globin with an efficiency comparable to those of HeLa and KB cell extracts. Using the procedure of Matsui et al. (T. Matsui, J. Segall, P.A. Weil, and R.G. Roeder, J. Biol. Chem. 255:11992-11996, 1980), we have isolated three active fractions from both MEL and HeLa cell extracts which are required for accurate transcription and have shown that equivalent fractions from MEL and HeLa cell extracts are interchangeable. Our findings suggest that the components required for initiation of transcription are similar in different cell types, at least to the extent that they can be assayed in these in vitro systems.

Distinct RNA polymerase II transcription preinitiation, initiation, and elongation complexes can be formed in vitro on cloned adenovirus 2 DNA sequences containing the major late promoter. These transcription complexes are stable and can be rapidly isolated by gel filtration of HeLa whole cell extracts. In the absence of exogenous nucleotides and under appropriate salt conditions, a stable but transcriptionally incomplete preinitiation complex is formed. When this complex is incubated in the presence of adenosine or deoxyadenosine triphosphates, the beta-gamma phosphodiester bond is hydrolyzed, and RNA polymerase II joins the complex, thereby converting it into a stable initiation complex capable of forming (but prior to the formation of) the first phosphodiester bond. When this complex is isolated and incubated in the presence of all four nucleoside triphosphates, it is converted into an elongation complex that then permits the synthesis of phosphodiester bonds and the correct run-off transcript. A limiting transcription component is sequestered in the preinitiation complex. This factor is released upon elongation and can reassociate with new DNA templates during subsequent rounds of initiation. Therefore, class II genes do not appear to form activated transcription units stable for multiple rounds of transcription; rather, their transcriptional activity may be controlled in part by regulating the association of transcription factors at each initiation event.

A two-step procedure has been developed for the formation of RNA polymerase II transcription initiation and elongation complexes. Initiation complexes are rapidly formed in HeLa cell-free extract supplemented with a DNA template containing the adenovirus 2 major late promoter and ATP. Assembly of transcription components required for correct initiation is absolutely dependent on specific eukaryotic promoter sequences. Sarkosyl-sensitive transcription initiation complexes are rapidly converted to Sarkosyl-resistant elongation complexes when supplemented with the remaining nucleoside triphosphates. The 60S initiation complex can be extensively purified by glycerol gradient centrifugation and is easily separated from free RNA polymerase II and free DNA template. Recovery of this stable complex is greater than 90%. Specific transcription cannot be detected if the DNA template is subsequently added to gradient fractions containing HeLa cell-free extract components alone. This suggests that the DNA templates promote the specific assembly of RNA polymerase II and transcription factors required for accurate initiation. Since conversion of purified initiation complexes to elongation complexes can occur without additional HeLa cell components, the presence of transcription components required for initiation and elongation in a single complex is indicated.