Spontaneous shuffling of domains between introns of phage T4

THE three self-splicing introns in phage T4 (in the td, sunY and nrdB genes) (Fig. 1a) each have the conserved group I catalytic RNA core structure (Fig. 1b), out of which is looped an open reading frame¹. Although the core sequences are very similar (̃60% identity), the open reading frames seem to be unrelated. Single crossover recombination events between homologous core sequences in the closely linked td and nrdB introns have led to 'exon shuffling'². Here we describe spontaneous double crossovers between the unlinked td and sunY introns that result in shuffling of an intron structure element, P7.1 (refs 3 and 4). The intron domain-switch variants were isolated as genetic suppressors of a splicing-defective P7.1 deletion in the td intron. This unprecedented example of suppression through inter-intron sequence substitution indicates that the introns are in a state of genetic flux and implies the functional Intel-changeability of the two analogous but non-identical P7.1 elements. The implications of such recombination events are discussed in the light of the evolution of the introns themselves as well as that of their host genomes.