An E. coli strain is Lac-negative because of a chromosomal deletion that removes the entire lac operon. When transformed with a plasmid vector carrying an antibiotic resistance gene and a 9.3 kb restriction fragment that contains the entire lac operon, the resulting transformant is able to grow on lactose. A spontaneous mutant was isolated from a culture of the transformant. The mutant was unable to grow on lactose, but was still antibiotic resistant. Plasmid DNA from this mutant was isolated. The restriction fragment containing the lac operon was found to be 10.5 kb in size.

a) Based on these results, what is the most probable cause of the mutation?

b) Further experiments reveal that the Lac-negative mutant (derived from the original transformant) reverts to Lac-positive phenotype at low frequency. Would you expect mutagens that cause either base-pair substitutions or single base-pair insertions and deletions to increase the frequency of Lac-positive revertants? Why?