Deletion during recombination in bacteriophage T7

The ligase gene of bacteriophage T7 was interrupted with inserts made from synthetic DNA. A pair of inserts were designed so that each insert contained one copy of an identical 17 bp sequence (repeat) positioned such that intermolecular recombination between the 17 bp homologies on separate genomes could delete enough of the insert to produce a functional ligase gene. The frequency of deletion by intermolecular recombination was compared to deletion frequency when repeated copies of the identical 17 bp sequence were present on the same genome separated by 39 bp. When the 17 bp homologous sequences were on different genomes the formation of ligase positive phage was about 7% to 13% of the deletion frequency measured with both repeats on the same genome. A second set of inserts contained the same 17 bp sequence present in the first pair of inserts. The sequence of this second set of inserts was such that when both 17 bp repeats were present on the same genome there was no separation between the repeats. With the second set of inserts (no separation) deletion by intermolecular recombination was about two orders of magnitude higher than what was measured with the first set of inserts where 39 bp of nonhomologous sequence intervened between the 17 bp repeats and the normal T7 genome. These data are interpreted to suggest that in T7 misalignment between repeated sequences during intermolecular recombination may play a role in deletion mutagenesis.