RNA splicing in the T-even bacteriophage.

Group 1 introns, first demonstrated in the nuclear large rRNA of Tetrahymena thermophila and subsequently in many yeast, fungal mitochondrial, and chloroplast precursor RNAs, are capable of intron excision and exon ligation in vitro, although this process occurs much more rapidly in vivo. The discovery and characterization of a similar intron in the T4 phage thymidylate synthase gene (td) led to the finding of additional group 1 introns in other T4 genes and in genes of the related T2 and T6 phages. Because protein factors are not required in the splicing of group 1 introns in vitro, it has been postulated that the precursor RNA can assume a critical conformation enabling it to undergo site-specific autocatalytic cleavage and ligation (self-splicing). By means of site-directed mutation, it has been shown unequivocally that several sequence elements in the Tetrahymena rRNA intron are involved in the formation of base-paired stem structures that are essential for the self-splicing process. These sequence elements have been demonstrated in other eukaryotic group 1 introns, as well as in the td intron. In this brief review we shall describe the biochemical and structural properties of the td intron in relation to other newly found phage introns. The interesting implications arising from these revelations will also be discussed.