Specificity of deletion events in pBR322

The reversion of mutations due to inserts of identical palindromic DNAs just 1-bp apart in the amp gene of plasmid pBR322 varied up to 3000-fold (U. DasGupta, K. Weston-Hafer, and D. E. Berg (1987) Genetics 115, 41-49). The experiments reported here show that the intrinsic frequencies of deletion from these sites are truly very different. Deletions were selected by the joint loss of sacB (sucrose sensitivity) and lacZα genes cloned together at these sites, without requiring restoration of the amp^r allele. We found that >90% of deletions at each of these sites do restore the amp^r allele. This result reinforces the view that the probability of forming a particular deletion depends strongly on the DNA sequence at its prospective endpoints.