EVOLUTION OF MOBILE GROUP-I INTRONS - RECOGNITION OF INTRON SEQUENCESBY AN INTRON-ENCODED ENDONUCLEASE

Mobile group I introns are hypothesized to have arisen after invasion by endonuclease-encoding open reading frames (ORFs), which mediate the ir mobility, Consistent with an endonuclease-ORF invasion event, we re port similarity between exon junction sequences (the recognition site for the mobility endonuclease) and intron sequences flanking the endon uclease ORF in the sunY gene of phage T4, Furthermore, we have demonst rated the ability of the intron-encoded endonuclease to recognize and cleave these intron sequences when present in fused form in synthetic constructs. These observations and accompanying splicing data are cons istent with models in which the invading endonuclease ORF is provided safe haven within a splicing element. In turn the intron is afforded i mmunity to the endonuclease product, which imparts mobility to the int ron.