DELETIONS INDUCED BY GAMMA-RAYS IN THE GENOME OF ESCHERICHIA-COLI

An Escherichia coli lysogen was constructed with a lambda phage bearing a lacZ gene surrounded by about 100 × 103 base-pairs of dispensable DNA. The lacZ mutants induced by gamma rays in this lysogen were more than 10% large deletions, ranging in size from 0·6 × 10-3 to 70 × 103 base-pairs. These deletions were centered, not on lacZ, but on a ColE1 origin of DNA replication located 1·2 × 103 bases downstream from lacZ. This suggested that this origin of replication was involved in the process by which the deletions were formed. In agreement with this hypothesis, a lysogen of the same phage without the ColE1 origin showed a very much lower percentage of radiation-induced deletions, as did a second lysogen of a lambda phage without any known plasmid origin of replication. Indirect evidence is presented for radiation-induced deletions centered on the lambda origin of DNA replication in a lysogen. It is suggested that high percentages of large deletions may occur among radiation-induced mutations in mammalian cells because deletions centered on some of the thousands of origins of replication in these genomes do not kill the cells. © 1991.