INTRON ORIGINS AND TRANSLATIONAL STREAMLINING

There is evidence that introns are basically ancient, though intron insertion events can also occur. Introns in the various classes of split gene may have been derived by ‘translational streamlining ’ from ancestral elements formerly performing functional roles in early cells, roles which later became redundant. For example, the exons of structural genes could have originally been small 'minigene ' mRNAs cotranscribed polycistronically, and the spliceosomal introns could formerly have been excised spacers or cleavage sites between the minigene cistrons. Similar ideas can be advanced for introns of tRNA and perhaps of rRNA. The intron problem The discovery of introns in the late 1970s was highly unexpected (Crick, 1979). Introns of various forms have been found in a variety of gene classes, not only in nuclear genomes, but also in non-nucleated genomes, those of organelles, mitochondrion and chloroplast, and to a degree even in prokaryote genomes. The origin of these strange intron sequences is a major area of debate. Here it is argued that introns are basicall