Regulation of Transposition
Because transposition may disrupt essential genes, excessive transposition would be lethal. To avoid this problem, most transposons carefully regulate the frequency of transposition. Typically the transposition frequency is enhanced when a transposon enters a naive host or immediately after DNA replication.
For example, transposition of Tn10 is controlled in several ways.
- Transposition is regulated by Dam methylation. Recall that D-adenine methylase (the dam gene product) adds a methyl group to the adenine residue in the sequence 5' GATC 3' ("dam sites"). Because the sequence is present in both strands of dsDNA, both strands of DNA are methylated within the paired GATC sequences. However, when the sequence is copied during DNA replication initially only the DNA strand derived from the parent is methylated. When one of the strands is methylated and the other is not, the site is hemimethylated. It takes about 1-2 min for the Dam to add a methyl group to the other strand to restore the fully methylated state.
Hemimethylated dam sites provide a signal that DNA replication has occurred. This signal is recognized by some DNA-binding proteins. In transposon Tn10 two dam sites play an important role in regulation of transposition.
- A dam site overlaps the -10 region of the promoter for transposase (Pin). When this dam site is fully methylated, RNA polymerase binds poorly to the promoter, limiting expression of transposase. When this dam site is hemimethylated, RNA polymerase binds to the promoter more efficiently, resulting in 10-fold increased expression of transposase. Increased expression of transposase increases the transposition of both Tn10 and IS10. (Recall that there is an IS10 located at each end of Tn10.)
- A dam site overlaps the transposase binding site on the inside ends of IS10 (but not the outside ends). When this dam site is fully methylated, transposase binds poorly to this site, limiting transposition of IS10. When this dam site is hemimethylated, transposase binds to this site more efficiently, resulting in 10-fold increased transposition of IS10. (Methylation of the dam site on the inside end of IS10 only affects IS10 transposition because when transposase binds to this site, it promotes transposition of IS10 but not Tn10.)
- Transposase preferentially acts in cis. If increasing the concentration of transposase increases the rate of transposition, why don't cells with multiple copies of a transposon show a higher frequency of transposition (precisely the condition I said that cells want to avoid)? There seem to be two ways to avoid this problem.
- Transposase is very unstable, so it is not normally accumulated to high concentrations.
- Rather than diffuse through the cell to find the end of a transposon, transposase probably binds to DNA immediately after it is made. Thus, the efficiency of transposition decreases as the distance between the transposase gene and its binding site (the ends of the transposon) increase. When the disance between the transposase gene and the end of transposon Tn10 is increased to 50 Kb (about 1% of the length of the E. coli chromosome, the frequency of transposition decreases about 15-fold.
- Transposase is repressed in trans. Translation of the Tn10 transposase gene is repressed by an antisense RNA as shown in the cartoon of below. (Recall that Tn10 is a composite transposon flanked by an IS element on the left side, IS10L, and on the right side, IS10R. Transposase is only encoded by IS10R, so the following figure only shows IS10R). The antisense transcript has a much longer half life then the transposase transcript, such that under steady state conditions there are about 5 copies of the antisense transcript in the cell and about 0.2 copies of the sense transcript. Thus, the antisense transcript ensures that the amount of transposase made by the cell remains quite low. In addition, this antisense transcript would repress expression of transposase from any Tn10 brought into a cell that already contains a copy of Tn10.
- Kleckner N., R. Chalmers, D. Kwon, J. Sakai, and S. Bolland. 1996. Tn10 and IS10 transposition and chromosome rearrangements: mechanism and regulation in vivo and in vitro.
Curr Top Microbiol Immunol. 204: 49-82.
- Labrador M., and V. Corces. 1997. Transposable element-host interactions: regulation of insertion and excision. Annu Rev Genet. 31: 381-404.
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Last modified November 3, 2003