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Mechanism Of Transposition Byberg

Mechanism Of Transposition Byberg
Berg (1977) has proposed a mechanism for the transposition of the Tn5 element. This element determines resistance to the antibiotic kanamycin, and can be transposed from the lambda phage to the E.coli chromosome, and from one chromosomal site to another. The frequency of transposition on the E. coli chromosome is between 10-2 to 10-3. There are many potential sites for the insertion of Tn5. The lacZ gene alone contains 19 sites. The revertibility of Tn5 mutations on lacZ gene is 19 out of 20, indicating that there is no loss of recipient DNA sequences during insertion.

Transposition consists of three phases, DNA cleavages, realign­ment of termini and ligation, and is a modification of the lambda phage integration mechanism. An enzyme complex' transposase ' recognizes and binds to specific sequences at the ends of the Tn5 element and also another DNA molecule with little or no sequence specificity. The enzyme complex brings about three DNA cleavages, one at each end of the junction of Tn5 DNA with vector DNA and one in the bound recipient molecule. The newly created termini of Tn5 and recipient DNA are realigned and ligated.

The freed vector fragments are released and degraded exonucleolytically. The basic diff­erence between. Tn5 integration and lambda phage integration is the absence in the former of sequence specificity of insertion, and of a closed, circular intermediate form in the recipient DNA molecule. The insertion mutation can undergo reversion. This can be looked upon as an abortive transposition event. Components from the transposase complex bind to the segment of DNA containing Tn5, cleave the DNA at C, and remove Tn5. Ligation of the ends near the sequences Land M restores the original sequence.

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