Bombyx mori silk protein genes are strictly turned on and off in different developmental stages under the hormone periodically change. The broad complex (BrC) is a transcription factor mediating 20-hydroxyecdysone action, which plays important roles during metamorphosis. Here, we observed that two isoforms of BmBrC (BmBrC-Z2 and BmBrC-Z4) exhibited contrasting expression patterns with fibroin genes (FibH, FibL and P25) in the posterior silk gland (PSG), suggesting that BmBrC may negatively regulate fibroin genes. Transgenic lines were constructed to ectopically overexpress BmBrC-Z2 in the PSG. The silk protein genes in the transgenic line were decreased to almost half of that in the wild type. The silk yield was decreased significantly. In addition, the expression levels of regulatory factors (BmKr-h1 and BmDimm) response to juvenile hormone (JH) signal were inhibited significantly. Then exogenous JH in the BmBrC-Z2 overexpressed lines can inhibit the expression of BmBrC-Z2 and activate the expression of silk protein genes and restore the silk yield to the level of the wild type. These results indicated that BmBrC may inhibit fibroin genes by repressing the JH signal pathway, which would assist in deciphering the comprehensive regulation mechanism of silk protein genes.

中文翻译：

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蚕体中宽复合物（BrC-Z2）的转基因异位过表达抑制家蚕的丝素蛋白基因的表达。

家蚕丝蛋白基因在不同发育阶段的严格开启和关闭激素下周期性变化。广泛的复合物（BrC）是介导20-羟基蜕皮激素作用的转录因子，在变态过程中起重要作用。在这里，我们观察到BmBrC的两个同工型（BmBrC-Z2和BmBrC-Z4）在后丝腺（PSG）中与纤维蛋白基因（FibH，FibL和P25）表现出相反的表达模式，这表明BmBrC可能会对纤维蛋白基因产生负调控。构建转基因品系以异位过表达BmBrC-Z2在PSG中。转基因品系中的丝蛋白基因减少到野生型中的几乎一半。蚕丝产量明显下降。此外，调节因子（BmKr-h1和BmDimm）对少年激素（JH）信号反应的表达水平也被显着抑制。然后，BmBrC-Z2过表达品系中的外源JH可以抑制BmBrC-Z2的表达并激活丝蛋白基因的表达，并使丝产量恢复至野生型。这些结果表明BmBrC可能通过抑制JH信号通路抑制丝蛋白基因，这将有助于破译丝蛋白基因的全面调控机制。