Development of a system useful for studying the formation of unstable alleles of IS2

IS2-induced deletions of the gal control region were isolated in a plasmid carrying gal OP-308::IS2-7. This contains a 54 basepair long, unstable mini insertion within IS2, thus allowing constitutive expression of the gal structural genes. Deletion PPI is 11.9 kilobasepairs (kb) long and is Gal+ because it has retained the mini insertion. In PP4 7.2 kb DNA material including markers gal OP, chlD and pgl are deleted. PP4 has lost the mini insertion and is therefore Gal negative. DNA sequencing of the newly formed junction in PP4 reveals that the deletion terminates precisely at nucleotide 1 of IS2 and that no DNA sequence homology is involved in this IS2-mediated deletion formation. PPI segregates Gal- clones due to the loss of the mini insertion. One such segregant PPIS and PP4 both give only constitutive Gal+ revertants, which consist of the previously known mini insertions and also a new class of "supermini" inserts within IS2 of about 10 to 20 basepairs long. Therefore, PPIS and PP4 can be used to study various parameters involved in the formation of mini insertions.