Evidence that introns arose at proto-splice sites

The unexpected discovery of introns raised many questions about gene evolution. We provide evidence that actin and tubulin introns were gained between the G and R of the conserved coding sequence C/AAGR that is known to flank introns in general and which we call a proto-splice site. We conclude that the tubulin and actin introns are less ancient than the coding sequence and so could not have been involved in the primary evolution of the tubulin and actin genes.