Two proteins in encoded by the E. coli lac operon are needed to metabolizes the

Question

Two proteins in encoded by the E. coli lac operon are needed to metabolizes the sugar lactose: the lactose permease (encoded bythe gene lacY) and the enzyme B-galactosidase (encoded by the gene lacZ). You have isolated a mutant E. coli strain, that you named lac, that cannot grow on plates where lactose is the only carbon sourse. What is the fastest and most efficient way to look for a new E. coli mutant that has lost the ability to utilize lactose as a source of energy ? Do a genetic selection by plating cells on minimal media plates with lactose (but no glucose), then look for the colonies that grow up. Do a genetic screen by growing colonies on minimal media plates with glucose (but no lactose), then replica plate to minimal media plates with lactose (but no glucose). Pick the colonies that fail to grow in the second media. Do a genetic screen by growing colonies on minimal media plates with lactose (but no glucose), then replica plate to minimal media plates with glucose (but no lactose). Pick the colonies that fail to grow in the second media. You take your lac strain and transform in three different F plasmids to conduct a complementation test. In the first experiment, you transform your lac bacteria with a F plasmid that contains the entire lac operon from a normal (wild-type) strain. In the second experiment, you transform your lac bacteria with a F plasmid that contains the lac operon from a strain missing the lacZ gene (AlacZ). In a third experiment, you transform your lac bacteria with a F plasmid that contains the lac operon from a strain missing the lacY gene (AlacY). Table 1 below shows the results obtained when the transformants were on plates with glucose or lactose as the sole carbone source. Growth of lac- transformants on glucose and lactose plates. What do you conclude from the previous experiment? the lac strain has a recessive mutation in the lacZ gene the lac strain has a dominant mutation in the lacZ gene the lac strain has a recessive mutation in the lacY gene the lac strain has a dominant mutation in the lacY gene the lac strain has a mutation in the lac repressor that prevents it from binding lactose; the mutant repressor never stops repressing transcription of lac operon even in the presence of lactose

Explanation / Answer

Q.24. B. Do a genetic screen by growing colonies on minimal media plates with glucose (but no lactose), then replica plate to minimal media plates with lactose(but no glucose). Pick the colonies that fail to grow in the second media.

Q25. C. the lac- strain has a recessive mutation in the lacY gene.

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