Introduction

In order to decipher the mechanisms that control the utilization of genetic information for time- and space-dependent expression we have been studying the developmental regulation of the silk protein genes of Bombyx mori (Suzuki, 1977; Suzuki et al., 1987, 1990b). The silk protein genes, which encode the protein components of the silk cocoon, are specifically transcribed in the silk gland and are temporally and spatially regulated during embryonic and larval development. The promoters/enhancers that are responsible for the highly specific expression patterns of the silk protein genes are expected to be correspondingly simpler than those specifying more complicated patterns of gene expression, such as embryonic patterning and segmentation (see Ingham, 1988, for a review), and correspondingly more tractable experimentally. Furthermore, the silk gland itself is highly amenable to biochemical experimentation and thus represents a facile model system for studying developmental gene activation of a group of differentiation-specific genes. We anticipate that studies of such differential gene activation will enable us to identify the regulatory components that underlie the events of cell differentiation and specialization.

Genetic analysis is a complementary approach that has recently provided a great deal of information about the general mechanisms governing development in Drosophila melanogaster and Caenorhabditis elegans. A group of developmental control genes, including those for membrane-bound receptors, proteases, growth factors, and transcription factors that form a temporally ordered regulatory cascade, has been identified.