This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. RNA molecules including ribozymes form another major class of structurally important biomolecules. Identifying the structure and folding mechanisms is critical for understanding their function. The goal of the proposal is to discern the important folding steps by identifying the structure of the folding intermediates. This information will used to further understand the function of this particular RNA, and folding and function of RNA molecules in general. Structural RNAs adopt 3D structures which are essential for their biological function. Predicting the 3D structure from the nucleotide sequence has proven remarkably difficult and remains a central problem in structural biology. The goal of this project is to understand the folding using time-resolved (sec) small-angle X-ray scattering (SAXS) methods. One of the fundamental structural parameters of a macromolecule is its over-all size and shape. SAXS can provide this information and is extremely well-suited for characterizing partially folded, short-lived states. The proposed measurements will be extremely useful in determining the sequence of events and, in particular the role of compaction in the folding process of the S-domain of the P RNA ribozyme. These data will be compared to those obtained using chemical modification and spectroscopy, and will greatly assist model building to produce a movie of the folding process. In particular, we wish to distinguish between four models based upon preliminary structural mapping data . These models have a Rg differing by 1  10 ¿, which is within the expected experimental accuracy.