Identification of the gal3 insertion in Escherichia coli AS IS2.

The gal3 mutation in Escherichia coli, located in the operator-promoter region of the gal operon, is identified as an IS2 insertion in the polar orientation I relative to the direction of transcription. This mutation, which may be considered the earliest example of a polar mutation caused by an IS insertion, is shown by heteroduplex analysis of phage lambdagal3 to be located about 170 base pairs from the promoter-proximal end of the chlD-pgl deletion in lambdagal8. It appears indistinguishable in position, sequence and orientation from the IS2 insertion carried by lambdagal8-490. The endpoints of the bacterial DNA segments in lambdagal3 and lambdagal8 are physically mapped in relation to attL.