You engineer a number of genetic mutations in E.coli's DNA and want to determine whether there is any change in size or relative abundance of the trytophanase operon mRNA produced by mutant strains. To do so, you will purify total mRNA from unmutated and mutated E.coli strains, run and separate the mRNAs on an agarose gel, and then perform a ___________ with a labelled nucleic acid probe to specifically visualize the tryptophanase mRNA.