The Ddx20/DP103 dead box protein represses transcriptional activation by Egr2/Krox-20

The early growth response 2 (Egr2/Krox- 20) transcription factor is essential for myelination of the peripheral nervous system and segmentation of the vertebrate hindbrain. To probe the mechanism by which Egr2 is regulated, we used a yeast two-hybrid assay and identified an RNA helicase, Ddx20 (DP103/Gemin3), as an Egr2-interacting protein. Mammalian two-hybrid assays indicated that Ddx20 can interact with Egr1, Egr3, and Egr4, in addition to Egr2, making it the only known cofactor that interacts with all four Egr family members. Using several Egr2 target promoters, we found that Ddx20 repressed Egr2-mediated transcriptional activation with significant promoter specificity. In addition, Ddx20 repressed Egr2-mediated activation of the endogenous insulin-like growth factor 2 (IGF2) gene. Interestingly, the C-terminal segment of Ddx20, which lacks the DEAD box helicase domain, was sufficient for its robust and specific repression. We also examined possible interactions between Ddx20 and Nab proteins, the only other known corepressors of the Egr family, and found that these two corepressors act independently. Finally, transcriptional repression assays performed in the presence of a histone deacetylase inhibitor ( trichostatin A) indicate that although repression of certain promoters by Ddx20 requires histone deacetylase activity, another repression mechanism must also be involved. Because Egr2 is critical for hindbrain development and peripheral nerve myelination, modulation of Egr2 by Ddx20 may play an important role in maintaining the correct expression level of Egr2 target genes.