You have isolated several E. coli mutants: Mutant #1 has a point mutation in the -10 region of the promoter of a structural gene encoding an enzyme needed for synthesis of the amino acid serine. Mutant #2 has a mutation in the -35 region in the promoter of the same gene. Mutant #3 is a double mutant with mutations in both the -10 and -35 region of the promoter of the same gene. Only Mutant #3 is unable to make serine. Why do you think this is so?