Evidence for a particular mode of transcription in globular loops of lampbrush chromosomes of the newt Pleurodeles waltlii.

In amphibian lampbrush chromosomes, many loops have a specific morphology; this is the case for globular loops in the newt Pleurodeles. We have previously shown that the specific morphology of these loops is linked to an extreme compactness of the transcription products which make up their matrix. We investigated RNA synthesis in this type of loop by carrying out autoradiographic and transcription inhibition studies. We also analysed the organization of transcriptional complexes in these loops in the electron microscope using spread preparations. These studies revealed the presence of several transcription units in the same loop and asynchronous variations in RNA synthesis in these transcription units. We propose and discuss several hypotheses in order to explain this asynchronous RNA synthesis. We also discuss these results in the context of loop morphology and transcription mode.