Ikaros interacts with P-TEFb and cooperates with GATA-1 to enhance transcription elongation.

Ikaros is associated with both gene transcriptional activation and repression in lymphocytes. Ikaros acts also as repressor of human γ-globin (huγ-) gene transcription in fetal and adult erythroid cells. Whether and eventually, how Ikaros can function as a transcriptional activator in erythroid cells remains poorly understood. Results presented herein demonstrate that Ikaros is a developmental-specific activator of huγ-gene expression in yolk sac erythroid cells. Molecular analysis in primary cells revealed that Ikaros interacts with Gata-1 and favors Brg1 recruitment to the human β-globin Locus Control Region and the huγ-promoters, supporting long-range chromatin interactions between these regions. Additionally, we demonstrate that Ikaros contributes to transcription initiation and elongation of the huγ-genes, since it is not only required for TBP and RNA Polymerase II (Pol II) assembly at the huγ-promoters but also for conversion of Pol II into the elongation-competent phosphorylated form. In agreement with the latter, we show that Ikaros interacts with Cyclin-dependent kinase 9 (Cdk9), which contributes to efficient transcription elongation by phosphorylating the C-terminal domain of the large subunit of Pol II on Serine 2, and favours Cdk9 recruitment to huγ-promoters. Our results show that Ikaros exerts dual functionality during gene activation, by promoting efficient transcription initiation and elongation.