Ultrastructural patterns of RNA synthesis during early embryogenesis of Drosophila melanogaster

Chromatin was obtained from Drosophila melanogaster during early embryogenesis and examined by transmission electron microscopy. Nuclear contents spread at progressive stages of syncytial development show a low level of only non-nucleolar template activity, and very few RNP fibril gradients extending over 2 μm in length are observed. At the cellular blastoderm stage, newly activated nucleolar genes appear during the early portion of the first true cell cycle. Variation in the lengths of incomplete rRNP gradients indicates that the activation of each rRNA gene is independently controlled. All rRNA loci, whether having complete or incomplete gradients, exhibit high densities of nascent transcripts per unit length, suggesting that the rate of chromatin transcription, rather than the RNA polymerase I pool size, limits rRNA synthesis on individual genes. No more than half the rRNA genes are derepressed at this stage, indicating that total rRNA synthesis is regulated by the number of genes activated. Non-nucleolar RNP fibril gradients covering up to 8 μm of genome are also first observed at the cellular blastoderm stage. Most of these gradients are differentiated from the short gradients first seen during syncytial growth by a lower density of transcribing RNA polymerases.