Although the expression of most genes is tightly regulated, some genes are expressed at roughly constant rates i.e., constitutively). Which of the following genes would you predict to be constitutively expressed?
A) genes involved in the biosynthesis of the amino acid tryptophan B) genes involved in the transport of the sugar maltose C) genes that code for ribosomal RNAs D) genes involved in the degradation of arabinose, a sugar E) genes involved in the degradation of tryptophan
A) lactose levels are high and glucose levels are low. B) lactose levels are low and glucose levels are high. C) lactose levels are high and glucose levels are high. D) lactose levels are low and glucose levels are low.
Imagine that you've set up a genetic screen to identify E. coli mutants that cannot metabolize the amino acid tryptophan. Beginning with a master plate containing many colonies, you prepare replica plates on medium with glucose or tryptophan as the only energy source. You would look for colonies that
A) can grow only on the plates with tryptophan. B) cannot grow on either type of media. C) can grow on both types of media. D) can grow only on the plates with glucose.