Movement by Conservative Transposition

The simplest transposons, including the insertion sequences, move by a mechanism known as conservative or "cut-and-paste" transposition (Fig. 15.04). This requires three elements: the transposase, the inverted repeats at the ends of the transposon, and a suitable target sequence on another segment of DNA somewhere else in the cell. The transposon may move to another site in the same molecule of DNA or to a separate molecule of DNA. The DNA molecule into which a transposon jumps can be a plasmid, a virus, or a chromosome; any DNA molecule will do, as long as it has a reasonable target sequence.

The transposase starts by binding the inverted repeats at the transposon ends. It then cuts the transposon loose from its original site (Fig. 15.05A). Next the transposase finds a suitable target sequence in the molecule of DNA that will be the transposon's new home. It makes a staggered cut that opens the target sequence to give overhanging ends (Fig. 15.05B). Finally, it inserts the transposon into the gap.

This leaves a structure with two short regions of single-stranded DNA. These are recognized by the bacterial host, which synthesizes the second strand. The net result is that the transposon has moved, and the target sequence has been duplicated in the process. This type of transposition process is known as conservative transposition because the DNA of the transposon is not altered during the move.

Where the transposon cut itself out of its original home, it leaves a double stranded break in the DNA. There is a significant likelihood that this damaged DNA molecule will not be repaired and is doomed. Clearly, high frequency transposition would severely damage the host cell chromosomes. Even if the break is sealed, the duplicated target sequence that is left behind may cause a permanent frameshift if within a coding sequence. Consequently, as remarked above, transposition must be tightly regulated.

conservative transposition Same as cut-and-paste transposition cut-and-paste transposition Type of transposition in which a transposon is completely excised from its original location and moves as a whole unit to another site

FIGURE 15.04 Outline of Conservative Transposition

The transposon moves from one DNA molecule to another. It inserts into the target sequence on the recipient DNA molecule and leaves behind a double-stranded break in its original location.


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