Molecular mechanism underlying juvenile hormone-mediated repression of precocious larval–adult metamorphosis

Abstract

Juvenile hormone (JH) represses precocious metamorphosis of larval to pupal and adult transitions in holometabolous insects. The early JH-inducible geneKrüppel homolog 1(Kr-h1) plays a key role in the repression of metamorphosis as a mediator of JH action. Previous studies demonstrated that Kr-h1 inhibits precocious larval–pupal transition in immature larva via direct transcriptional repression of the pupal specifierBroad-Complex(BR-C). JH was recently reported to repress the adult specifier geneEcdysone-induced protein 93F(E93); however, its mechanism of action remains unclear. Here, we found that JH suppressed ecdysone-inducibleE93expression in the epidermis of the silkwormBombyx moriand in aB. moricell line. Reporter assays in the cell line revealed that the JH-dependent suppression was mediated by Kr-h1. Genome-wide ChIP-seq analysis identified a consensus Kr-h1 binding site (KBS, 14 bp) located in theE93promoter region, and EMSA confirmed that Kr-h1 directly binds to the KBS. Moreover, we identified a C-terminal conserved domain in Kr-h1 essential for the transcriptional repression ofE93. Based on these results, we propose a mechanism in which JH-inducible Kr-h1 directly binds to the KBS site upstream of theE93locus to repress its transcription in a cell-autonomous manner, thereby preventing larva from bypassing the pupal stage and progressing to precocious adult development. These findings help to elucidate the molecular mechanisms regulating the metamorphic genetic network, including the functional significance ofKr-h1,BR-C, andE93in holometabolous insect metamorphosis.