The chromatin constituents associated with transcriptionally active genes can be studied by a variety of experimental approaches. Most of the methods used involve the isolation of active chromatin fractions followed by biochemical analysis, or are based on in vitro assays with the ultimate goal of reconstituting faithful transcription by adding purified and defined components to the transcriptional machinery (for reviews see Mathis, Oudet & Chambon, 1980; Rose, Stetler & Jacob, 1983a; Beroldingen et al. 1984; Reeves, 1984; Dynan & Tjian, 1985; Sentenac, 1985). Since these approaches require isolation of chromatin or of individual components therefrom, possible structural rearrangements or selective losses of certain constituents cannot a priori be excluded. Therefore it often remains questionable whether the results obtained in vitro reflect the actual situation in the living cell.

In order to examine the nature of transcriptionally active chromatin in the living cell and to decipher the in vivo function of nuclear proteins in transcriptional processes, we have microinjected antibodies to various nuclear antigens directly into the nucleus of living amphibian oocytes.

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