Abstract
The organization of transcription of a well-characterized protein encoding gene was studied by microinjection and electron microscopy. Circular recombinant DNA molecules containing the complete chicken ovalbumin sequences (7.7 kilobases, contained in 11.5 kilobases of chicken DNA) were microinjected into germinal vesicles of living oocytes of the clawed toad Xenopus laevis, and their transcription was studied in nuclear spread preparations. Evaluation of spread chromatin showed a limited number of observed molecules transcribed in "specific" patterns--i.e., circular chromatin molecules containing transcription units approximately 2.3 micrometer long, consisting of regular series of densely packed lateral ribonucleoprotein fibrils gradually increasing in length. The appearance of these fibril gradients was similar to that of actively transcribed endogenous protein encoding genes contained in lampbrush-chromosome loops of the same nuclei and to the putative Bombyx silk fibroin transcription units. In addition, less-regular arrays of transcript fibrils were seen in some circles, including fully fibril-covered molecules, indicative of the occurrence of irregular transcriptional events. The results of this heterologous transcription experiment indicate that the transcriptional machinery of the amphibian oocyte nucleus is capable of transcribing protein encoding genes from an avian species in typical regular arrays of transcription units.