The Primary Structure of tRNAs and Their Rare Nucleosides

Abstract

Since the first determination of a primary structure of a tRNA by Holley et al. (1965), 113 structures have been published of which 108 are different. This number is sometimes difficult to calculate. Two structures are defined as different if they differ by one basic nucleotide, for example, if an A replaces a G, but not if they differ by posttranscriptional modifications. Some structures have been determined on mixtures of isoacceptors that were impossible to separate, like the one of beef liver tRNA^Trp (Fournier et al. 1978). Figure 1 shows that this tRNA has several incompletely modified nucleotides, such as m²G in the amino acid stem, Cm in the anticodon loop, and m⁷G in the variable loop. But in two positions, C is partially replaced by D. If these Ds arise from a posttranscriptional modification of C, then we would have only two tRNAs^Trp, one having a G in position 56 and the other having an A in this position. In fact, so far, only U → D heterogeneities have been found in tRNAs. There is no evidence that a C can be modified to a D. Therefore, we conclude that two more basic nucleotides are different in the tRNA^Trp. There are several combinations possible between tRNAs differing in three different positions; we have at least three tRNAs^Trp. In fact, by two-dimensional gel electrophoresis, five spots have now been found for this tRNA (M. Fournier, pers. comm.). It will be interesting to test if each functions as primer for reverse...