INTRON ORIGINS AND TRANSLATIONAL STREAMLINING

Abstract 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 inearly 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 orcleavage,sites between ,the minigene ,cistrons. Similar ideas can be advanced ,for introns of tRNA and perhaps of rRNA.