REGIONAL SPECIFICITY OF ILLEGITIMATE RECOMBINATION BY THE TRANSLOCATABLE AMPLICILLIN-RESISTANCE ELEMENT Tn1 IN THE GENOME OF PHAGE P22

Insertions of the translocatable ampicillin-resistance element Tnl were selected in the genome of the temperate Salmonella phage P22 by growing the phage on hosts carrying the resistance plasmid RP4. Insertions of Tnl into phage P22 are rare (10-10 per phage) and nonrandomly distributed in the P22 genome. They are found mainly in the vicinity of the P22 ant gene. Insertions within the ant gene are found at many (at least 15) genetically separable sites, are found equally frequently in both orientations and cause irreversible loss of gene function. Some insertions in ant appear to be associated with an adjecent deletion.—Prophage deletions were derived from P22::Tnl phages by two methods. Low multiplicity transductants have nonrandomly distributed endpoints. One end is at or very near the site of the Tnl insertion, and the other is in the vicinity of gene 12; however, there are many genetically distinguishable endpoints within gene 12. Prophage deletions selected as survivors of induction of a P22Ap mnt-ts lysogen have similarly nonrandom endpoints, with the Tnl-distal end frequently near the ant gene, as well as gene 12. Physical analysis of several prophage deletions suggests that the Tnl is intact to the resolution of DNA electron microscopy and that the deletions begin at the end of the Tnl insertion.—These results suggest that illegitimate recombination associated with Tnl shows regional specificity (i.e., preference for some large areas of the P22 genome over other areas), but that within these regions is quite nonspecific.