title: Topological repression of gene activity by a transposable element.
abstract: The ebgA (evolved beta-galactosidase) gene of Escherichia coli was isolated as part of a 9.6-kilobase (kb) sequence cloned into plasmid pBR322. The position of the ebgA gene within that 9.6-kilobase sequence was identified by insertional inactivation by means of the transposon gamma-delta. In addition to the gamma-delta insertions that inactivate ebgA by disrupting the coding sequence, seven additional gamma-delta insertions reduce expression of the gene by a factor of greater than 200 by insertions elsewhere into the replicon. One of these insertions is into the pBR322 sequence itself. This action at a distance to reduce expression requires that gamma-delta is cis with respect to the ebgA gene. The effect is independent of the orientation or position of gamma-delta within the replicon, but it does depend both upon the orientation of the ebgA-bearing sequence within the replicon and upon the total size of the replicon. Transcription readthrough (promoter occlusion) does not explain this phenomenon, and we suggest that the presence of gamma-delta may alter the local supercoiling in the region of the ebgA promoter in such a way as to inhibit transcription. This repression by a transposable element appears to represent a novel mechanism for altering gene expression.
mesh_terms: D004262:DNA Restriction Enzymes; D004251:DNA Transposable Elements; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D005696:Galactosidases / Q000235:genetics*; D005796:Genes; D005798:Genes, Bacterial; D005838:Genotype; D010641:Phenotype; D010957:Plasmids; D012093:Replicon; D013489:Suppression, Genetic; D001616:beta-Galactosidase / Q000235:genetics*
chemical_list: D004251:DNA Transposable Elements; D004262:DNA Restriction Enzymes; D005696:Galactosidases; D001616:beta-Galactosidase