A Family of Autocatalytic Group I Introns in Bacteriophage T4

The first intron to be discovered in a prokaryotic mRNA was found in the td gene of bacteriophage T4 (see Belfort et al., this volume). The presence of an intron in this gene, which encodes thymidylate synthase, was detected by sequence comparisons at the DNA and protein levels. Thus, a stop codon was encountered in the DNA sequence before the end of the protein-coding sequence (Chu et al. 1984). Subsequent experimentation revealed that the intron in the td gene was excised autocatalytically from precursor RNA (Belfort et al. 1985). The splicing mechanism resembled that used by group I introns of eukaryotes: a series of transesterification reactions triggered by nucleophilic attack by guanosine (or GTP) at the 5′ splice site (Ehrenman et al. 1986; Chu et al. 1987). The primary intron excision product was linear, containing a non-coded G at the 5′ end (Ehrenman et al. 1986).