A bacterium has the following lac operon genes located on the chromosome: I+P?Z?Y?A+. On a plasmid, this same bacterial cell has the lac genes I?P+Z?Y+A?. Which protein products will be made from lac genes in response to the presence of lactose?
What type of protein expression (induced or constitutive) will there be for each protein that is made?
What will be an ideal response?
Answer: Based on this information the lac repressor protein R will be made and will be functional in the absence of lactose. However, the lac operon on the chromosome does not have a functioning promoter; thus only the plasmid-encoded copy will be transcribed efficiently in the presence of lactose. The plasmid contains only one normal lac gene, lac Y; therefore, the cell will make only galactoside permease and will not be able to use lactose for growth and energy production.
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