21 tRNA Splicing: An RNA World Add-on or an Ancient Reaction?

Abstract

Introns are encoded in the genes for tRNA in organisms from all three kingdoms of life. Their removal is an essential step in the maturation of tRNA precursors. In Bacteria, introns are self-splicing and are removed by a group 1 splicing mechanism (Kuhsel et al. 1990; Reinhold-Hurek and Shub 1992; Biniszkiewicz et al. 1994). In Eukaryotes and Archaea, intron removal is mediated enzymatically by proteins. Recent progress in understanding both eukaryotic and archaeal tRNA splicing has revealed that the two processes, previously thought to be unrelated, are in fact similar. Insight gained from the comparison has provided a clearer understanding of intron recognition, the catalysis of intron removal, and has given new insight into the evolution of the tRNA splicing process.

INTRON-CONTAINING tRNA
tRNA Introns in Eukaryotes
Intervening sequences (introns) were discovered 20 years ago in the yeast genes for the tyrosine-inserting non-sense suppressor tRNA (Goodman et al. 1977) and for tRNA^Phe (Valenzuela et al. 1978). With the completion of the yeast genome, it is known that of the 274 yeast tRNA genes 61, or 20%, contain introns. Table 1 lists the tRNAs that contain introns. PCR cloning of tRNAs from higher eukaryotes has revealed a similar distribution of intron-containing tRNA (Stange and Beier 1986; Green et al. 1990; Schneider et al. 1993). The introns in all of the genes are small (14–60 bases), and they are all located in the same position, one base to the 3′ side of the anticodon (Fig. 1A). Structure probing revealed that the...