Metal ion-induced changes in HIV-1 TAR RNA internal dynamics were determined by the changes in EPR spectral width for TAR RNAs containing spin-labeled nucleotides (U23, U25, U38, and U40). This gave a dynamic signature for each of 10 metal ions studied, which fell into one of three distinct groups. While Li+ and K+ had little effect on TAR RNA internal dynamics, Na+ unexpectedly had a dynamic signature that was similar to Ca2+ and Sr2+ with a decrease in mobility at U23 and U38, little or no change at U25, and an increase in mobility at U40. Mg2+, CO2+, Ni2+, Zn2+, and Ba2+ had similar effects on U23, U38, and U40, but the mobility of U25 was markedly increased. Our results show that RNA dynamics change upon metal binding to the TAR RNA bulge, indicating that RNA structure adapts to accommodate metal ions of different size and coordination properties. (C) 2003 Elsevier Science (USA). All rights reserved.