Clover leaf model

The secondary clover-leaf structure of tRNA is stabilized by Watson-Crick base pairs. The tRNA s are a large family of molecules consisting of 71 to 76 nucleotides, with about 10% of rare bases (Fig. 15.2). The known nucleotide sequences of over 200 tRNA s [695J can be arranged in a characteristic clover-leaf model with four double helical stems and three loop regions. In some of the positions, the same nucleotide occurs, called invariant, in others only the type is conserved, i.e.,... 

Fig. 1. Clover leaf model of a tRNA molecule (serine-specific tRNA from yeast). A = adenine. C = cytosine. G guanine. I = inosine. U = uracii. T = thymine, tj = pseudouracii.

The structure of tRNA has been estabUshed in greater detail. These naturally occurring nucleic acids are the smallest so far isolated and in many cases the sequence has been determined. The clover leaf model, first proposed by Holley et al. (1965) is now generally accepted as a description of secondary structure in these biologically important nucleic acids. The tertiary structure has not been completely elucidated but it is clear that the molecule is extremely compact and is composed of double-helical regions with hairpin loops containing five to seven nucleotides. The structure is quite stable and is probably more compact than that of rRNA. 

Work on tRNA yields a of 23-2.5 nm by X-ray scattering. Many of the earliest studies predated the crystal structure determination of tRNA in 1974 [347-355] and thus attempted to elucidate its overall shape on the basis of the clover-leaf base-pairing model from sequence data. Open structures were discarded in favour of more compact schemes. Many folded structures were, however, compatible with the experimental data, while a simple triaxial body was not. The observation of two distinct i xs values led to the development in 1970 of a model with one large and two small ellipsoids whose main axes are parallel to one another and are arranged in an L-shape [353], and which anticipated the L-shape determined by crystallography (Fig. 26). Other studies on tRNA have investigated its melting... 

Fig. 10. Structural model of serine tRNA. Many rare bases are conspicuous in the clover leaf structure, e.g. I = inosine or IPA (page 205) directly adjacent to the anticodon (modified from Zachau et al. 1966).

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