Developmental regulators containing the I-mfa domain interact with T cyclins and Tat and modulate transcription.

Positive transcription elongation factor b (P-TEFb) complexes, composed of cyclin-dependent kinase 9 (CDK9) and cyclin T1 or T2, are engaged by many cellular transcription regulators that activate or inhibit transcription from specific promoters. The related I-mfa (inhibitor of MyoD family a) and HIC (human I-mfa-domain-containing) proteins function in myogenic differentiation and embryonic development by participating in the Wnt signaling pathway. We report that I-mfa is a novel regulator of P-TEFb. Both HIC and I-mfa interact through their homologous I-mfa domains with cyclin T1 and T2 at two binding sites. One site is the regulatory histidine-rich domain that interacts with CDK9 substrates including RNA polymerase II. The second site contains a lysine and arginine-rich motif that is highly conserved between the two T cyclins. This site overlaps and includes the previously identified Tat/TAR recognition motif of cyclin T1 required for activation of human immunodeficiency virus type 1 (HIV-1) transcription. HIC and I-mfa can serve as substrates for P-TEFb. Their I-mfa domains also bind the activation domain of HIV-1 Tat and inhibit Tat- and P-TEFb-dependent transcription from the HIV-1 promoter. This transcriptional repression is cell-type specific and can operate via Tat and cyclin T1. Genomic and sequence comparisons indicate that the I-mf and HIC genes, as well as flanking genes, diverged from a duplicated chromosomal region. Our findings link I-mfa and HIC to viral replication, and suggest that P-TEFb is modulated in the Wnt signaling pathway.