Properties of DNA Insertion Elements of E. coli
The presence of IS elements in the chromosome of E.coli was originally revealed by the transposition of these DNA elements from their natural positions into indicator systems, resulting in a recognizable mutant phenotype. If, for example, the gal operon is used as an indicator system, mutations can be isolated in which not only one of the three structural genes of the galactose operon is inactivated, but the expression of the Promoter distal genes is also abolished. These mutations therefore are strongly polar. The analysis of the nature of such mutations is facilitated by the existence of the gal transducing phage λ and the development of techniques for examining heteroduplex DNA in the electron microscope. With the help of these tools it is possible to inspect hybrid DNA molecules consisting of one DNA strand carrying the strongly polar mutation and a complementary strand of λ dgal in the electron microscope. The strongly polar mutation is seen as a single stranded DNA loop emerging from a position of the double stranded heteroduplex molecule, which corresponds to the map position of the mutation. Analysing various independently isolated strongly polar mutations with the above technique, revealed the existence of different categories of IS elements.
KeywordsInsertion Sequence Indicator System Natural Component Polar Mutation Inverted Orientation
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