Lac messenger RNA in lac Z gene mutants of Escherichia coli caused by insertion of bacteriophage Mu

The messenger RNA made under conditions of induction of the lac operon has been studied in various lac mutants in which the mutation was caused by integration of bacteriophage Mu into the Z gene. The percentage of RNA hybridizing specifically to lac DNA is proportional to the distance from the beginning of the gene at which a given mutation is located. It thus appears that only lac RNA proximal to the site of insertion is transcribed in these mutants. This may account for the complete polarity of Mu-induced lacZ mutants.