Transposons usually cause strongly polar mutations, preventing transcription of downstream genes in an operon. It is possible to take advantage of this to determine the order of genes in an operon. An example of an operon with a MudJ operon fusion is shown below. (The red lines indicate the mRNA transcript.)
Both the A::Tn10 and B::Tn10 insertions are polar on expression of the lac operon fusion. Thus, the phenotypes of these mutants indicate that both the A and B gene are upstream of the C::MudJ fusion. Although the order is shown in the cartoon, you could not infer the order of the A and B genes by simply looking at the phenotypes. To determine the order of these two genes, you would need to determine the phenotype of an A::MudJ fusion with and without a B::Tn10 insertion, and the phenotype of an B::MudJ fusion with and without a A::Tn10 insertion.
It is essential to consider the polarity of transposon insertions when interpreting the phenotype of a transposon insertion mutant, because the phenotype may be due to polarity on a downstream gene, not the gene that contains the transposon insertion. A simple test to distinguish these possibilities is to do complementation analysis.
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Last modified October 14, 2003