Transposition may Rearrange Host DNA

Insertions, deletions and inversions of host cell DNA may result from anomalous or failed attempts at movement by a composite transposon. As remarked above, the trans-posase will move any segment of DNA surrounded by a pair of correct inverted repeats. Composite transposons have four such terminal repeats. Two of these are "inside ends", relative to the transposon, and two are "outside ends" (Fig. 15.10 above). Any pair that face in opposite directions may be used together. Transposition of a single IS element involves one "inside end" and one "outside end". Normal movement of the whole composite transposon uses the two "outside ends".

But suppose that the two "inside ends" are used for transposition. The whole of the DNA molecule outside the transposon will be moved. This is easiest to see if we

Transposition may Rearrange Host DNA 409

FIGURE 15.11 Evolution of a Composite Transposon

Since composite transposons no longer require two separate transposase genes and four inverted repeats, mutations accumulate in the non-essential regions. These mutations ensure that the transposon moves as a composite unit. This is especially important if the transposon carries internal genes that enhance the survival of the host cell.

Non-functional

Inactive Functional inverted inverted repeat repeat

Non-functional

Inactive Functional inverted inverted repeat repeat

FIGURE 15.11 Evolution of a Composite Transposon

Since composite transposons no longer require two separate transposase genes and four inverted repeats, mutations accumulate in the non-essential regions. These mutations ensure that the transposon moves as a composite unit. This is especially important if the transposon carries internal genes that enhance the survival of the host cell.

Functional inverted repeat Mutations a<-

FIGURE 15.12 Insertion Created by Using Inside Ends to Transpose

Plasmid DNA can integrate into a chromosome if the "inside ends" (b and c) of the composite transposon are moved by transposase. Inverted repeats b and c point outwards from the transposon, but the transposase is still capable of moving the DNA between them— i.e., the DNA making up most of the plasmid. If the transposase gene is located inside the transposon (in the purple segment), the DNA that jumped will not be able to move itself again, i.e., it is not itself a genuine transposon.

^tansposofl

Degraded

Inside ends of IS sequences (i.e. b and c)

are used in transposition b a dc

This is NOT a transposon, as it has no transposase gene consider a small circular DNA molecule, such as a plasmid (Fig. 15.12), that carries a composite transposon. Using the "outside ends" moves the transposon, using the "inside ends" moves the rest of the DNA molecule. [Notice that the "inside ends" and the "outside ends" face in different directions.] The result, in the case illustrated, is the insertion of a segment of plasmid DNA into the host chromosome. Note that the segment that moved may not be able to move again if the gene for the transposase was left behind during this maneuver.

If both of the "inside ends" are used in jumping to another site on the same host DNA molecule the result will be the insertion of the host DNA into itself (Fig. 15.13). Depending on which pairs of ends are re-joined after breakage, the host DNA will suffer either a deletion or an inversion. The inside region of the composite transposon is lost during this process. This process is sometimes known as abortive transposition.

FIGURE 15.13 Deletions and Inversions made by Abortive Transposition

"Inside ends" (b and c) are incorrectly used by transposase to move the host DNA rather than the transposon. If the target sequence (p-q) is found on the same DNA molecule, transposase cuts this site also, creating a double-stranded break. There are two alternative ways of rejoining the ends. Separate molecules may be formed, thus creating a deletion in the host DNA molecule. Alternately, the small piece (p-b) may rejoin the larger fragment such that region p joins IS sequence c-d, and region q joins IS sequence a-b, thus inverting the DNA.

-ttansppso^ Degraded a

INVERSION (of region from b to p)

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