Chapter 14, Problem 28P

A researcher has a Trp auxotrophic strain of E. coli with a mutation in a single gene. To identify that mutant

gene, she uses a genomic library made from a wild-type version of that same strain to find plasmids that rescue the mutant phenotype. The result is surprising. She recovers 10 plasmids that provide a Trp phenotype, but six

of the plasmids contain gene X, while the other four contain gene Y. Our scientist has encountered a

phenomenon called multicopy suppression, related to the fact that plasmids are usually present in several copies per bacterium. Because the genes in the plasmids are present in more than their usual single copy in the

bacterial chromosome, more than the usual amount of Protein X or Protein Y is being produced from the plasmids. Sometimes, overexpression of one protein can rescue the mutant phenotype caused by loss of a

different protein. Suggest at least two ways that our scientist could determine which of the two genes, gene X or gene Y, actually corresponds to the mutant gene causing the Trp phenotype.