7 NEWLY DISCOVERED INTRON POSITIONS IN THE TRIOSE-PHOSPHATE ISOMERASEGENE - EVIDENCE FOR THE INTRONS-LATE THEORY

The gene encoding the glycolytic enzyme triose-phosphate isomerase (TP I; EC 5.3,1,1) has been central to the long-standing controversy on th e origin and evolutionary significance of spliceosomal introns by virt ue of its pivotal support for the introns-early view, or exon theory o f genes. Putative correlations between intron positions and TPI protei n structure have led to the conjecture that the gene was assembled by exon shuffling, and five TPI intron positions are old by the criterion of being conserved between animals and plants. We have sequenced TPI genes from three diverse eukaryotes-the basidiomycete Coprinus cinereu s, the nematode Caenorhabditis elegans, and the insect Heliothis vires cens-and have found introns at seven novel positions that disrupt prev iously recognized gene/protein structure correlations. The set of 21 T PI introns now known is consistent with a random model of intron inser tion. Twelve of the 21 TPI introns appear to be of recent origin since each is present in but a single examined species. These results, toge ther with their implication that as more TPI genes are sequenced more intron positions will be found, render TPI untenable as a paradigm for the introns-early theory and, instead, support the introns-late view that spliceosomal introns have been inserted into preexisting genes du ring eukaryotic evolution.