Differential usage of two promoters of the Broad-Complex gene in the silkworm, Bombyx mori

Krüppel Homolog 1 Inhibits Insect Metamorphosis via Direct Transcriptional Repression of Broad-Complex, a Pupal Specifier Gene

The Broad-Complex gene (BR-C) encodes transcription factors that dictate larval-pupal metamorphosis in insects. The expression of BR-C is induced by molting hormone (20-hydroxyecdysone (20E)), and this induction is repressed by juvenile hormone (JH), which exists during the premature larval stage. Krüppel homolog 1 gene (Kr-h1) has been known as a JH-early inducible gene responsible for repression of metamorphosis; however, the functional relationship between Kr-h1 and repression of BR-C has remained unclear. To elucidate this relationship, we analyzed cis- and trans elements involved in the repression of BR-C using a Bombyx mori cell line. In the cells, as observed in larvae, JH induced the expression of Kr-h1 and concurrently suppressed 20E-induced expression of BR-C. Forced expression of Kr-h1 repressed the 20E-dependent activation of the BR-C promoter in the absence of JH, and Kr-h1 RNAi inhibited the JH-mediated repression, suggesting that Kr-h1 controlled the repression of BR-C. A survey of the upstream sequence of BR-C gene revealed a Kr-h1 binding site (KBS) in the BR-C promoter. When KBS was deleted from the promoter, the repression of BR-C was abolished. Electrophoresis mobility shift demonstrated that two Kr-h1 molecules bound to KBS in the BR-C promoter. Based on these results, we conclude that Kr-h1 protein molecules directly bind to the KBS sequence in the BR-C promoter and thereby repress 20E-dependent activation of the pupal specifier, BR-C. This study has revealed a considerable portion of the picture of JH signaling pathways from the reception of JH to the repression of metamorphosis.

Ecdysone response elements in the distal promoter of the Bombyx Broad-Complex gene, BmBR-C

The Bombyx mori silkworm's homologue of the Broad-Complex gene (BmBR-C) is transcribed from two promoters: a distal promoter (Pdist) and a proximal promoter (Pprox). As determined by a luciferase assay, the transcriptional activity of Pdist, but not Pprox, was activated by ecdysone. Further analyses using reporters driven by sequential deletion Pdist mutants indicated that two regions, ecdysone responsive element (EcRE)-D and EcRE-P, -4950 bp and -3480 bp upstream from the distal transcription start site, respectively, were important in the responsiveness of Pdist to 20-hydroxyecdysone (20E); however, no significant sequence similarities were found between the canonical EcRE and the EcRE-D or EcRE-P regions. Electrophoretic mobility shift assays showed that both the EcRE-D and -P sequences specifically bound to Bombyx protein(s). Sequence analyses and competition assays suggested that the protein(s) bound to EcRE-P might include components other than the ecdysone receptor (EcR), suggesting that BmBR-C transcription was indirectly activated by ecdysone through the EcRE-P. Remarkably, protein binding to the mid-region of the EcRE-D, EcRE-Db, was competitively inhibited by an oligonucleotide containing the Drosophila hsp27 EcRE sequence. Furthermore, an anti-EcR antibody interfered with the formation of the protein-EcRE-Db complex. These results indicated that a functional Bombyx ecdysone receptor binds to EcRE-D and activates the expression of BmBR-C.

Stage-specific activation of the E74B promoter by low ecdysone concentrations in the wing disc of Bombyx mori

To understand the transcriptional regulation of E74B by low concentrations of ecdysone, the promoter activity of Bombyx mori E74B was assessed in the B. mori wing disc using a transient reporter assay. We identified the transcription start sites of BmE74B and found that the core promoter region consists of initiator (Inr) and downstream promoter elements (DPE). The 3.6-kb upstream promoter region of BmE74B was responsive to 20-hydroxyecdysone (20E) in a dose-dependent manner, and the highest luciferase activity was observed in the presence of 0.2 μg/ml 20E. Moreover, the upstream BmE74B promoter activity was induced by 20E in a stage-specific and time-dependent manner, and the 3.6-kb promoter contained essential elements for the temporal regulation of BmE74B. Furthermore, we found a set of putative ecdysone response elements (EcREs). Five of these elements are highly conserved, capable of binding to the ecdysone receptor. Mutation of more than three putative EcREs, followed by introduction into the wing discs, abolished the activation of the BmE74B promoter by a low concentration of ecdysone. The results confirmed the role of ecdysone response elements in the transcription regulation of BmE74B and demonstrated that multiple putative EcREs were involved in the maximum response of BmE74B to low concentrations of ecdysone.