Background

The silk gland of Bombyx mori, the commercial silkworm, provides one of the most striking examples of regulated polymerase III action. Study of this lepidopteran transcription system thus offers the possibility of insight, into a particularly interesting aspect of polymerase III function. Moreover, it should illuminate basic features of class III transcription, and given recent indications of mechanistic similarity between polymerases II and III (Lobo and Hernandez, 1989; Murphy, Moorefield, and Pieler, 1989; Lobo, Lister, Sullivan, and Hernandez, 1991; Margottin et al., 1991), it could reveal even more fundamental aspects of eukaryotic transcription.

The fact that polymerase III activity is regulated is not always obvious because many of the polymerase III products are ubiquitous RNAs, such as 5S ribosomal RNA and tRNA. Although it is true that both 5S RNA and tRNA are synthesized by all cells, it is clear that the levels of particular species of these general RNA types vary widely. For instance, the amounts of oocyte type 5S RNA are strikingly different in oocytes and somatic cells of Xenopus frogs (Brown and Sugimoto, 1973), and the tRNA population of reticulocytes changes during differentiation to match the requirements for globin synthesis (Hatfield, Matthews, and Rice, 1979; Hatfield, Varricchio, Rice, and Forget, 1982). Specialization of the tRNA population in the Bombyx mori silk gland is particularly dramatic (see Hui and Suzuki, this volume, for a description of B. mori silk glands). Here, the correlation between amino acid usage and the machinery responsible for protein synthesis is especially clear because of the unusually simple amino acid composition of the major product of the posterior silk gland, fibroin.