Genes VII

Some transposons carry drug resistance (or other) markers in addition to the functions concerned with transposition. These transposons are named Tn followed by a number. One class of larger transposons are called composite elements, because a central region carrying the drug marker(s) is flanked on either side by "arms" that consist of IS elements.

The arms may be in either the same or (more commonly) inverted orientation. So a composite transposon with arms that are direct repeats has the structure

Arm L → Central Region Arm R→

If the arms are inverted repeats, the structure is

Arm L→ Central Region Arm R←

The arrows indicate the orientation of the arms, which are identified as L and R according to an (arbitrary) orientation of the genetic map of the transposon from left to right. The structure of a composite transposon is illustrated in more detail in Figure 15.2, which also summarizes the properties of some common composite transposons.

Since arms consist of IS modules, and each module has the usual structure ending in inverted repeats, the composite transposon also ends in the same short inverted repeats.

In some cases, the modules of a composite transposon are identical, such as Tn9 (direct repeats of IS1) or Tn903 (inverted repeats of IS903). In other cases, the modules are closely related, but not identical. So we can distinguish the L and R modules in Tn10 or in Tn5.

A functional IS module can transpose either itself or the entire transposon. When the modules of a composite transposon are identical, presumably either module can sponsor movement of the transposon, as in the case of Tn9 or Tn903. When the modules are different, they may differ in functional ability, so transposition can depend entirely or principally on one of the modules, as in the case of Tn10 or Tn5.

We assume that composite transposons evolved when two originally independent modules associated with the central region. Such a situation could arise when an IS element transposes to a recipient site close to the donor site. The two identical modules may remain identical or diverge. The ability of a single module to transpose the entire composite element explains the lack of selective pressure for both modules to remain active.

What is responsible for transposing a composite transposon instead of just the individual module? This question is especially pressing in cases where both the modules are functional. In the example of Tn9, where the modules are IS1 elements, presumably each is active in its own right as well as on behalf of the composite transposon. Why is the transposon preserved as a whole, instead of each insertion sequence looking out for itself?

Two IS elements in fact can transpose any sequence residing between them, as well as themselves. Figure 15.3 shows that if Tn10 resides on a circular replicon, its two modules can be considered to flank either the tet^R gene of the original Tn10 or the sequence in the other part of the circle. So a transposition event can involve either the original Tn10 transposon (marked by the movement of tet^R) or the creation of the new "inside-out" transposon with the alternative central region.

Note that both the original and "inside-out" transposons have inverted modules, but these modules evidently can function in either orientation relative to the central region. The frequency of transposition for composite transposons declines with the distance between the modules. So length dependence is a factor in determining the sizes of the common composite transposons.

A major force supporting the transposition of composite transposons is selection for the marker(s) carried in the central region. An IS10 module is free to move around on its own, and mobilizes an order of magnitude more frequently than Tn10. But Tn10 is held together by selection for tet^R; so that under selective conditions, the relative frequency of intact Tn10 transposition is much increased (for review see Kleckner, 1989).

The IS elements code for transposase activities that are responsible both for creating a target site and for recognizing the ends of the transposon. Only the ends are needed for a transposon to serve as a substrate for transposition.