Activation of the mouse TCRγ enhancers by STAT5

The IL-7R controls local accessibility of joining (J) γ gene segments in the mouse TCRγ locus by recruiting signal transducers and activators of transcription (STAT) 5 and transcriptional coactivators to the Jγ germ line promoters and inducing histone acetylation and germ line transcription. Because STAT consensus motifs are conserved not only in the Jγ promoters but also in the TCRγ 3′ enhancer (Eγ) elements, it is possible that STAT5 interacts with and activates Eγ. To address this question, we first showed that the lysine 4 residue of histone H3 is substantially methylated at Eγ1 and Eγ4 elements in wild-type early thymocytes and that the levels of the methylation are reduced in IL-7R α chain-deficient mice. We also showed that STAT5 has potential to elevate histone acetylation of the Eγ elements in a cytokine-dependent cell line by cytokine stimulation. Next, we demonstrated that STAT5 is recruited to the STAT consensus motifs in the Eγ elements after cytokine stimulation and that transcription factors Runt-related (Runx) and c-Myb are constitutively recruited to Eγ. Furthermore, we showed that STAT5 augments basal Eγ activity controlled by Runx and c-Myb. These results suggest that STAT5 is recruited to the consensus motifs in the Eγ elements by cytokine stimulation and augments basal Eγ activity independent of Runx and c-Myb. Therefore, this study implies that the Eγ elements might be activated in two successive steps, first by Runx and c-Myb and next by STAT5.