Uracil incorporation in the forespore and the mother cell during spore development in Bacillus subtilis. Autoradiographic electron microscopic study.

The transcriptional activity of the two genomes of the sporangium during spore formation was determined by pulse-labeling bacteria with 3H-uracil at different times of sporulation and preparing them for high resolution autoradiography. The quantitative analysis of autoradiographs shows that uracile incorporation in the whole sporangium decreases considerably between stages II and IV. However, the variations of the transcriptional activity are not identical in the mother cell and in the forespore. The one of the mother cell decreases rapidly between stages II and III and then remains stable until the end of stage IV, whereas that of the forespore which is low at stage II increases as the forespore grows ovoid and then quickly diminishes. It is very weak at the beginning of stage IV and negligible at the end of this stage. Pulse-chase experiments made in the presence of rifampine indicate that about 30% of the uracile incorporated is located in stable RNA. This value is found at any stage of sporulation in both cellular compartments whatever their rate of uracile incorporation. A relationship can be made between the nuclear shape and the activity of the genetic material. This confirms observations made by several authors in other bacterial species and other physiological conditions that the condensed shape corresponds to a state of low transcriptional activity whereas the more irregular and dispersed shape corresponds to a state of high activity.