NMR analysis of the trans-activation response (TAR) RNA element of equine infectious anemia virus.

Transcription of lentiviral DNA in the host cell is regulated by an interaction between the viral TAR RNA stem-loop and the viral Tat protein. Here we present a model of the three-dimensional structure of the TAR RNA stem-loop of the equine infectious anemia virus (EIAV), derived from two- and three-dimensional NMR data. This 25 nucleotide RNA consists of an A-form helical stem capped by two U-G base pairs and a four-nucleotide loop. Two loop cytidines are stacked into the loop interior and likely form a non-Watson-Crick C-C base-pair. The two nucleotides at the top of the loop, U13 and G14, appear to be excluded from the interior of the loop and solvent exposed. It is significant that now for the EIAV TAR-Tat system, three-dimensional structures are now known for both the RNA and protein components.