Visualization of the Silk Fibroin Transcription Unit and Nascent Silk Fibroin Molecules on Polyribosomes of Bombyx mori

This chapter reviews that the unique physical properties of the silk fibroin gene of Bombyx mori, its complementary mRNA molecule and the fibroin polypeptide have allowed biochemical probes to define the kinetics of fibroin production. Late in larval development, the highly polyploid posterior silk gland cells contribute over 80% of their protein synthesis to the production of silk fibroin. By modifying chromosome spreading techniques first adapted for visualizing extrachromosomal nucleolar genes of amphibian oocytes, it examines the transcriptional and translational organization of silk producing Bombyx cells. It discusses that adequately dispersed Bombyx posterior silk-gland genomes show: (1) inactive “beaded chromatin regions, (2) active ribosomal genes, (3) variously sized non-nucleolar genes that are typically populated with low densities of RNA polymerase molecules, and (4) a distinct population of ribonucleoprotein fibril gradients between 5 and 6 μm long that are packed with almost as many RNA polymerases per unit length as the rRNA genes, but are not present in tandem array. The distribution of the three categories of ribonucleoprotein fibril gradients changes as larvae proceed through the fifth instar, with the category of long, polymerase-dense gradients becoming more prominent. The chapter observes that a specific structural eukaryotic gene has been visualized and the first visual confirmation of the accepted biochemical interpretation of protein synthesis.