text: For a considerable time following the first tRNA structures, the field of RNA structure did not dramatically advance.  The ability to study an RNA structure depended upon the potential to isolate the RNA target.  This proved limiting to the field for many years, in part because other known targets—i.e., the ribosome—were significantly more difficult to isolate and crystallize.  Further, because other interesting RNA targets had simply not been identified, or were not sufficiently understood to be deemed interesting, there was simply a lack of things to study structurally.  As such, for some twenty years following the original publication of the tRNA structure, the structures of only a handful of other RNA targets were solved, with almost all of these belonging to the transfer RNA family.  This unfortunate lack of scope would eventually be overcome largely because of two major advancements in nucleic acid research: the identification of ribozymes, and the ability to produce them via in vitro transcription.
  Subsequent to Tom Cechs publication implicating the Tetrahymena group I intron as an autocatalytic ribozyme, and Sidney Altmans report of catalysis by ribonuclease P RNA, several other catalytic RNAs were identified in the late 1980s, including the hammerhead ribozyme.  In 1994, McKay et al. published the structure of a hammerhead RNA-DNA ribozyme-inhibitor complex at 2.6 Ångström resolution, in which the autocatalytic activity of the ribozyme was disrupted via binding to a DNA substrate. The conformation of the ribozyme published in this paper was eventually shown to be one of several possible states, and although this particular sample was catalytically inactive, subsequent structures have revealed its active-state architecture.  This structure was followed by Jennifer Doudnas publication of the structure of the P4-P6 domains of the Tetrahymena group I intron, a fragment of the ribozyme originally made famous by Cech. The second clause in the title of this publication—Principles of RNA Packing—concisely evinces the value of these two structures: for the first time, comparisons could be made between well described tRNA structures and those of globular RNAs outside the transfer family.  This allowed the framework of categorization to be built for RNA tertiary structure.  It was now possible to propose the conservation of motifs, folds, and various local stabilizing interactions.  For an early review of these structures and their implications, see RNA FOLDS: Insights from recent crystal structures, by Doudna and Ferre-DAmare.
  In addition to the advances being made in global structure determination via crystallography, the early 1990s also saw the implementation of NMR as a powerful technique in RNA structural biology.  Coincident with the large-scale ribozyme structures being solved crystallographically, a number of structures of small RNAs and RNAs complexed with drugs and peptides were solved using NMR. In addition, NMR was now being used to investigate and supplement crystal structures, as exemplified by the determination of an isolated tetraloop-receptor motif structure published in 1997. Investigations such as this enabled a more precise characterization of the base pairing and base stacking interactions which stabilized the global folds of large RNA molecules.  The importance of understanding RNA tertiary structural motifs was prophetically well described by Michel and Costa in their publication identifying the tetraloop motif: "...it should not come as a surprise if self-folding RNA molecules were to make intensive use of only a relatively small set of tertiary motifs.  Identifying these motifs would greatly aid modeling enterprises, which will remain essential as long as the crystallization of large RNAs remains a difficult task".