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Methoprene-tolerant 1 (Met1) is a basic-helix-loop-helix Per/Arnt/Sim (bHLH-PAS) protein identified as the intracellular receptor of juvenile hormone (JH). JH induces phosphorylation of Met1; however, the phosphorylation site and outcomes of phosphorylation are not well characterized. In the present study, using the lepidopteran insect and serious agricultural pest Helicoverpa armigera (cotton bollworm) as a model, we showed that JH III induced threonine-phosphorylation of Met1 at threonine 393 (Thr393) in the Per-Arnt-Sim (PAS) B domain. Thr393-phosphorylation was necessary for Met1 binding to the JH response element (JHRE) to promote the transcription of Kr-h1 (encoding transcription factor Krüppel homolog 1) because Thr393-phosphorylated Met1 increased its interaction with Taiman (Tai) and prevented the Met1-Met1 association. However, JH III could not prevent Met1-Met1 association after Met1-Thr393 was mutated, suggesting that Thr393-phosphorylation is an essential mechanism by which JH prevents Met1-Met1 association. The results showed that JH induces Met1 phosphorylation on Thr393, which prevents Met1-Met1 association, enhances Met1 interaction with Tai, and promotes the binding of Met1-Tai transcription complex to the E-box in the JHRE to regulate Kr-h1 transcription. Image 1 • JH III induces Met1 phosphorylation on Thr393. • Met1 phosphorylation on Thr393 enhances its interaction with transcription factor Taiman. • Thr393-phosphorylation was necessary for Met1 binding to the JH response element. • Met1 phosphorylation on Thr393 prevents Met1-Met1 association. [ABSTRACT FROM AUTHOR]