A STEM-LOOP KISSING MODEL FOR THE INITIATION OF RECOMBINATION AND THEORIGIN OF INTRONS

Mutations which improve the efficiency of recombination should affect either the proteins which mediate recombination or their substrate, DN A itself. The former mutations would be localized to a few sites. The latter would be dispersed. Studies of hybridization between RNA molecu les have suggested that recombination may be initiated by a homology s earch involving the ''kissing'' of the tips of stem loops. This predic ts that, in the absence of other constraints, mutations which assist t he formation of stem loops would be favored. From comparisons of the f olding of normal and shuffled DNA sequences, I present evidence for an evolutionary selection pressure to distribute stem loops generally th roughout genomes. I propose that this early pressure came into conflic t with later local pressures to impose information concerning specific function. The conflict was accommodated by permitting sections of DNA concerned with a specific function to evolve in dispersed segments. T races of the conflict seem to be present in some modern intron-contain ing genes, Thus, introns may have allowed the interspersing of selecti vely advantageous stem loops in coding regions of DNA.