MYB transcriptional elongation is regulated by an attenuator sequence within intron 1 that has been proposed to encode a RNA stem loop (SLR) followed by a polyU tract. We report that NF kappa Bp50 can bind the SLR polyU RNA and promote MYB transcriptional elongation together with NF kappa Bp65. We identified a conserved lysine-rich motif within the Rel homology domain (RHD) of NF kappa Bp50, mutation of which abrogated the interaction of NF kappa Bp50 with the SLR polyU and impaired NF kappa Bp50 mediated MYB elongation. We observed that the TAR RNA-binding region of Tat is homologous to the NF kappa Bp50 RHD lysine-rich motif, a finding consistent with HIV Tat acting as an effector of MYB transcriptional elongation in an SLR dependent manner. Furthermore, we identify the DNA binding activity of NF kappa Bp50 as a key component required for the SLR polyU mediated regulation of MYB. Collectively these results suggest that the MYB SLR polyU provides a platform for proteins to regulate MYB and reveals novel nucleic acid binding properties of NF kappa Bp50 required for MYB regulation.
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