Suppose that gene X is induced in E. coli cells when they are treated with high levels of ethanol. A mutation in region 1 in the upstream region of gene X results in high-level expression under basal conditions, and a mutation in region 2 results in an uninducible gene. What would you expect from a strain in which both regions 1 and 2 are mutated?

Because there is a mutation in region one that causes high level expression even in the absence of ethanol, region 1 must bind a transcriptional repressor and regulation of gene X must occur by relief of repression. Since a mutation in gene 2 results in a uninducibility of the gene, region 2 must bind a transcriptional activator. in the absenece of an activator the reprssor becomes irrelevant and the gene is in active regadless of conditions. Mutations in both regions 1 and 2 have the same effect as a single mutation in region 2, because in the absence of a bound transcriptional activator the repressor is irrelaevnat and the gene is inactive regardless of the presence or absence of ethanol