Searching for mutations
When researchers reveal how mutants overproduce antibiotics, the real work has only just begun--successful reverse engineering hinges on the ability to reproduce the mechanism of overproduction in new strains. Ideally, the exact sequence of base pairs causing the mutation could be found and then directly introduced into a new strain.
Drug companies use chemicals and radiation to damage DNA and induce mutations. But it's not easy to detect these single base-pair mutations in the so-called industrial mutant strains. Kao's group, for example, detected the presence of an unidentified regulator gene altering the timing of erythromycin production, but could not pinpoint the specific mutation.
A similar study by Kao's group identified two genes responsible for overproduction of the antibiotic tylosin. Instead of increasing the length of time that a bacterium produces the antibiotic--the case with erythromycin--these two genes allow the bacterium to produce it faster. Again, the specific mutations eluded Kao's group.
Currently, the researchers are trying to identify the mutations by chopping overproducer DNA into small pieces, several genes in length, and inserting them into normal bacteria to look for changes in either expression of antibiotic gene clusters or final antibiotic yields.
Studying mutant strains that already exist only represents half of Kao's work; she also creates her own mutants. Creating the mutants on her own offers two advantages. First, instead of making mutants with chemicals and radiation, her group uses transposons--small DNA segments that can replicate on their own and insert their copies into new positions anywhere in the genome. Mutations induced by transposons are significantly easier to identify.
A technique called DNA shuffling, recently developed by the Redwood City-ba