Various colorimetric indicators have been developed that allow one to easily dis

Question

Various colorimetric indicators have been developed that allow one to easily distinguish between gal + and gal –. These can be used, for example, to measure mutagenesis rates of a gal + specialized transducing phage that carries the gal genes. e.g., Tetrazolium in the appropriate media will cause gal + to make a red plaque on an appropriate E. coli host, compared to a colorless plaque for gal -. To measure mutagenesis rates of the E. coli host bacteria, there are many assays that can easily be applied (e.g., the rate of colonies that appear that are resistant to an antibiotic.).

An E. coli culture is lightly irradiated with UV light such that ~98% of the cells are still able to make colonies. Among the survivors, we find that the rate of mutation to antibiotic resistance has increased 50-fold relative to an un-irradiated culture. Interestingly, if we grow (un-irradiated) gal + on the irradiated and un-irradiated cells, the rate of gal – plaques is also increased 50-fold by the irradiated host even though the phage saw no damaging UV! What is the explanation of this effect?

Explanation / Answer

The lambda phage has shown specialized transduction in which after initial insertion, the lambda gal looped out through homology dependent recombination. Looping out removes the gal gene and thus, the lambda phage progeny become deficient of the gene.

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