Genetic Delineation of Functional Components of the Group I Intron in the Phage T4 td Gene

Group I introns are extremely widespread, occurring in nuclear or organellar genes of a variety of plants, slime molds, protists, and fungi as well as in bacteriophage genomes (for review, see Cech 1986). The classification of these evolutionarily diverse introns into a single group was based on their ability to adopt a similar core secondary structure (Davies et al. 1982; Michel et al. 1982; Michel and Dujon 1983). It is believed that this distinctive RNA structure imparts the catalytic character to these molecules (Cech et al. 1983; Michel and Dujon 1983; Waring and Davies 1984), which play an instrumental role in their own splicing reactions. Regardless of whether these reactions proceed spontaneously or require accessory factors, group I splicing occurs by a common mechanism (Cech 1986; Garriga and Lambowitz 1986). The distinguishing feature of the splicing pathway, which proceeds by a series of transesterification reactions, occurs at its first step....