Relative orientation of RNA helices in a group 1 ribozyme determined by helix extension electron microscopy.

The relative orientation of helical elements in a folded RNA molecule provides key information about its three-dimensional architecture. We have developed a method that involves extending peripheral helices of an RNA, mounting for electron microscopy in the absence of protein and measuring interhelical angles. As a control, extended anticodon and acceptor stems of tRNA(Phe) were found to form a 92 +/- 20 degrees angle, consistent with the X-ray structure. Single, double and triple extensions (50-80 bp) of helical elements P2.1, P6b and P8 of the Tetrahymena group I ribozyme did not alter its catalytic activity. The measured angle between P6b and P8 is consistent with the Michel-Westhof structural model, while the P2.1-P6b and P2.1-P8 angles allow P2.1 to be positioned in the model. The angle distributions of the ribozyme are broader than those of the tRNA, which may reflect the dynamics of the RNA. Helix extension allows low-resolution electron microscopy to provide much higher resolution information about the disposition of helical elements in RNA. It should be applicable to diverse RNAs and ribonucleoprotein complexes.