Insights into the regulatory characteristics of silkworm fibroin gene promoters using a modified Gal4/UAS system

The silkworm Bombyx mori is a valuable insect that synthesizes bulk amounts of fibroin protein in its posterior silk gland (PSG) and weaves these proteins into silk cocoons. The mechanism by which the fibroin protein is efficiently synthesized and precisely regulated is an important aspect that has yet to be fully elucidated. Here, we describe the regulatory characteristics of the promoters of fibroin protein-encoding genes, namely, fibroin heavy chain (fibH) and fibroin light chain (fibL), using an optimized Gal4/UAS binary system. We found that (1) UAS-linked enhanced green fluorescent protein (EGFP) was effectively activated in the PSGs of Gal4/UAS transgenic silkworms, and fluorescence was continuously detected in the PSGs after complete formation of silk glands. (2) In the PSGs of fourth- and fifth-instar larvae of transgenic silkworms driven by fibL-Gal4 (LG4) or fibH-Gal4 (HG4), EGFP mRNA was detected in only day-3 to day-6 fifth-instar larvae, while the EGFP protein could be detected at each day of both larval stages. (3) High-level expression of Gal4 and UAS-linked EGFP caused a delay in PSG degradation in Gal4/UAS transgenic silkworms. (4) At the early pupal stage, EGFP fluorescence was also detected in fat bodies of Gal4/UAS transgenic silkworms, indicating that the PSG-specific EGFP was transported into fat bodies during PSG degeneration; however, the underlying mechanism needs to be further elucidated. This study provides a modified Gal4/UAS system used for efficient tissue-specific expression of target genes in the PSGs of silkworms and provides new insights into the regulatory characteristics of the promoters of key fibroin protein-encoding genes.