Question

You have found a new streptomycin resistance allele in Streptococcus (the same bacteria that Avery and Griffiths were working with). A wild type version of this allele is normally found within the strain you are working in (hence it’s normally streptomycin sensitive). When you isolate genomic DNA from the strepR strain then transform the original Streptococcus strain with this plasmid, because of natural transformation, you can isolate strep resistant Streptococcus colonies. When you PCR up this same gene and transform Streptococcus (including all relevant promoter regions), you can never select for resistant colonies. Describe one potential explanation for these experimental results

Answer 1

One potential explanation for these experimental results:

When you isolate genomic DNA from the strepR strain then transform the original Streptococcus strain with this plasmid, because of natural transformation, you can isolate strep resistant Streptococcus colonies. But you can never select for resistant colonies because plasmid DNA is non-methylated and thereby not protected from restriction digestion in wild type hosts.

While transformation, when the plasmid containing the streptomycin resistant gene entered into the host cell that is wild type version[ streptomycin sensitive] then there are many restriction enzymes present in the host cell which may cleave the streptomycin resistant gene in the plasmid as they are non-methylated and hence these genes are degraded and they fail to confer the antibiotic resistance property to the wild type host cell and hence no resistant colonies were found.