Exon coconversion biases accompanying intron homing: battle of the nucleases

Intron homing in phage T4 occurs in the context of recombination-dependent replication, by virtue of intron-encoded endonucleolytic activity. After the td intron endonuclease I-TevI cleaves the intronless recipient 23 and 25 nucleotides upstream of the intron insertion site, exonucleolytic degradation is required for recombination to proceed. This resection process results in coconversion of exon sequences flanking the intron. In a genetic system designed to study coconversion of flanking markers, we demonstrate that although there is a bidirectional polarity gradient, coconversion can be highly asymmetric. Furthermore, we show that the coconversion of flanking markers favors exon I sequences, upstream of the I-TevI cleavage site. These data are consistent with the asymmetric features of the homing pathways that have been invoked for intron mobility in phage T4. Moreover, these results are in accord with the finding that once the td homing-site substrate is cleaved, I-TevI remains bound to the downstream cleavage product, protecting against exonucleolytic degradation, and thereby limiting the extent of coconversion into exon II. The results suggest that recombination events are influenced by a competition between the homing endonuclease and exonucleases for sequences downstream of the I-TevI cleavage site, thereby implying a role for the homing endonuclease in the repair process.