Structural studies on the RNA-recognition motif of NELF E, a cellular negative transcription elongation factor involved in the regulation of HIV transcription

The elongation of transcription of HIV RNA at the TAR (transactivation-response element) is highly regulated by positive and negative factors. The cellular negative transcription elongation factor NELF (negative elongation factor) was suggested to be involved in transcriptional regulation of HIV-1 (HIV type 1) by binding to the stem of the viral TAR RNA which is synthesized by cellular RNA polymerase II at the viral long terminal repeat. NELF is a heterotetrameric protein consisting of NELF A, B, C or the splice variant D, and E. In the present study, we determined the solution structure of the RRM (RNA-recognition motif) of the RNA-binding subunit NELF E and studied its interaction with the viral TAR RNA. Our results show that the separately expressed recombinant NELF E RRM has a-helical and P-strand elements adopting a beta alpha beta beta alpha beta fold and is able to bind to TAR RNA. Fluorescence equilibrium titrations with fluorescently labelled double- and single-stranded oligoribonucleotides representing the TAR RNA stem imply that NELF E RRM binds to the single-stranded TAR RNAs with K-d values in the low-micromolar range.