Structure-function relations in E. coli 16S RNA.

As a more thorough understanding of RNA secondary and tertiary structure has developed over the last several years, so has an appreciation of its importance in the function of the various RNP particles of which it is a part. For example, Ul RNA has been linked to mRNA splicing (Lerner et al., 1980; Rogers and Wall, 1980) while the RNA moiety of RNAase P has been shown to be absolutely required for its activity (Kole et al., 1980). RNA has even been shown to be capable of making and breaking phosphodiester bonds in the complete absence of protein (Kruger et al., 1982). Along with these developments, the concept that ribosomal RNA is merely a framework on which ribosomal proteins can carry out their functions has been discarded. Indeed, speculations on the evolution of the protein-synthesizing system have generally concluded that the RNA must have predated the protein components. The similarity in structure of protein-free 16s RNA in solution and 16s RNA in the 30s subunit (observed with psoralen crosslinking by Wollenzein et al., 1979; Thammana et al., 1979; Thompson and Hearst, 1983; and with electron microscopy) suggests that at least vestiges of the original catalytic structure remain. While Escherichia coli rRNA may no longer be able to carry out protein-free translation, it is now generally accepted that it plays an active role in ribosomal functions, Unfortunately, the dearth of structural information has allowed formulation of only simple models for how RNA operates. Even though the sequence of 16s RNA is known (Brosius et al., 1978; Carbon et al., 1979) and much of its secondary structure is agreed on (Noller and Woese, 1981; Stiegler et al., 1981; Zwieb et al., 1981) little progress has been made towards linking specific structures with function. Recent work with psoralen crosslinking of 16s RNA (Thompson and Hearst, 1983) has confirmed parts of the secondary structure, and has also provided evidence for new interactions that appear to be functionally important. We will discuss how these structural features may be related to specific ribosomal mechanisms. We will concentrate on E. coli 16s RNA, but eucaryotic 18s RNA will also be presented when its function appears to be substantially different. Reference to most ribosomal proteins will be tastefully omitted, primarily because their interactions with the RNA are poorly understood, but also because we have approached the problem with the bias that they modulate the activity of the RNA rather than being the principal driving force behind it.