Gene 2 is incomplete Gene 3 is not transcribed

Gene 2 is incomplete Gene 3 is not transcribed their presence blocks transcription of any other downstream genes that share the same promoter as the gene that suffered the insertion event. This effect is referred to as polarity. Since bacterial genes are often found clustered in operons and are co-transcribed onto the same mRNA (see Ch. 6), they are much more likely than eukaryotic genes to show polarity effects due to insertions.

Occasionally, insertions may activate genes. If an insertion occurs in the recognition site for a repressor, binding of the repressor will be prevented and activation of the gene may result. In addition, a few transposons are known to have promoters close to their ends, facing outwards (Fig. 13.08). Insertion of these may activate a previously silent gene. Examples are known of "cryptic" genes that have potentially functional gene products that cannot be expressed due to defective promoters. Thus the bgl operon of E. coli is inactive in the wild type and only expressed in mutants when a transposon carrying an outward-facing promoter is inserted just upstream of the operon and reactivates it.

polarity When the insertion of a segment of DNA affects the expression of downstream genes, usually by preventing their transcription

DNA Rearrangements Include Inversions, Translocations, and Duplications 343



DNA Promoter Gene

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