5 IDENTICAL INTRON POSITIONS IN ANCIENT DUPLICATED GENES OF EUBACTERIAL ORIGIN

IN 1985 Cornish-Bowden wrote ''although there is now much to suggest t hat introns are an ancient relic of primordial genes, convincing proof must await the discovery of clearly corresponding intron arrangements in genes that arose by duplication before the separation of prokaryot es and eukaryotes''1. Genes for chloroplast and cytosolic glyceraldehy de-3-phosphate dehydrogenases of eukaryotes are descendants of an anci ent gene family that existed in the common ancestor of extant eubacter ia. During eukaryotic evolution, both genes were transferred to the nu cleus from the antecedents of present-day chloroplasts and mitochondri a. respectively2-5. Here we report the discovery of five spliceosomal introns at positions that are precisely conserved between nuclear gene s for this chloroplast/cytosol enzyme pair. These data provide strong evidence in favour of the 'introns early' hypothesis, which proposes t hat introns were present in the earliest cells, consistent with the id ea that introns facilitated the assembly of primordial genes by accele rating the rate of exon shuffling6-13.