Original articles Modeling the 3-D RNA distribution in the Balbiani Ring granule

Mature Balbiani Ring (BR) granules in situ were stained with the nucleic acid specific stain, osmium ammine-B, recorded by electron spectroscopic imaging and reconstructed by electron microscope tomography to examine the three-dimensional (3-D) distribution of BR heterogeneous nuclear RNA (hnRNA). The BR2 gran- ules contain ca. 37 kb of mRNA. Reconstructed BR granules were selected to emphasize one of the prevalent conformations seen in the sectioned salivary glands, the enface or "pin-wheel" conformation. A variety of image processing and volume-rendering operations were ap- plied to the set of reconstructed BR granules. Some of the conclusions of this study are the following: (1) RNA distribution is not uniform throughout the granule; (2) RNA is condensed into about ten particles per granule, which all appear to possess approximately the same RNA stain density; (3) heterogeneity exists in the posi- tions and sizes of particles within the various BR gran- ules. These data argue for the folding of a beaded rib- bon, consisting of connected particulate condensations of BR mRNA, possessing considerable 3-D flexibility, even in the packaged state. A comparison of this beaded- ribbon model and a prior folded hnRNP fiber model is also presented.