Gene regulation

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1. A scientist wants to study the regulation of the cbbA gene (whose gene product function is unknown). He has a strain with a deletion of the chromosomal cbbA gene and containing a low copy number plasmid with a cbbA:lacZ transcriptional fusion (the strain therefore lacks a functional cbbA gene). While -galactosidase activity in this strain is low, spontaneous mutants appear with high activity. In these mutants, he replaces the fusion plasmid with a similar one carrying cbbA+ (using an incompatible plasmid with a different, selectable drug-resistance), and analyze the level of cbbA product. Rationalize the following possible results:

Case A. The cbbA product is present at a very high level.

Case B. He detects approximately wild-type levels of the cbbA gene product.

Case C. Drug-resistant transformants of the second plasmid are hard to get and, when analyzed, all make little or no cbbA+ product.

I want to know the explanation for this three points