The sericin-1 gene encoding a glue protein is expressed in the middle silk gland (MSG) of the silkworm, Bombyx mori. A member of the class III POU domain transcription factors, POU-M1, was cloned as the factor bound to the SC site of the sericin-1 promoter and has been proposed to be a positive transcription factor. In this study, we analyzed the expression pattern of

Protein modifications with highly conserved small proteins, such as ubiquitin (Ub) and small ubiquitin‐like modifier (SUMO), regulate various cellular processes; however, the contribution of these protein modifications to larval development in insects has not yet been elucidated. We investigated the regulation of genes for these protein modifications in the posterior silk gland (PSG) during larval development of the silkworm Bombyx mori. We found that several genes encoding enzymes (E1, E2, and E3) for ubiquitination and SUMO‐specific protease were upregulated by 20‐hydroxyecdysone (20E), and, consistently, increases in ubiquitinated proteins were observed during the fourth molting stage. An injection of 20E into larvae at the fourth feeding stage induced higher expression levels of these E1, E2, and E3 genes and ecdysis approximately one day earlier than in mock‐treated larvae. The expression of the fibroin heavy‐chain gene (fibH) was simultaneously suppressed approximately one day earlier in 20E‐injected larvae. The treatment of cultured PSG with 20E also induced these genes, which could be categorized into at least two types: those induced by a high dose of 20E, or by a pulse of 20E. In contrast to the 20E treatment, the administration of PR‐619, an inhibitor of Ub‐ and SUMO‐specific proteases in larvae, delayed ecdysis and prolonged the expression of fibH. These results suggest that the regulation of genes for ubiquitination and SUMO‐specific protease is involved in the larval development of B. mori.