Many bacteria, including E.coli, have a lac operon to enable them to digest lactose when glucose is not available. The lac operon contains a promoter and an operator followed by the genes lacZ, lacY and lacA.
You plate the same strain of E.coli in two different types of media, A and B. You measure expression of the lac genes and find that they are strongly expressed in media A and very weakly expressed in media B.
Which of the following statements is most likely to be correct regarding the compositions of media A and media B respectively?