Reaction Reversal by ResT

Like its bacteriophage counterparts (Huang et al. 2004a, ResT appears not to turn over during the course of the in vitro reaction. This raised the possibility of very slow product release or product inhibition of resolution. In the course of investigating this issue ResT was found to be able to interact with its hp telomere products. Unexpectedly, ResT is also able to cleave its product and to fuse hp telomeres together in a chemical reversal of the resolution reaction (Kobryn and Chaconas 2005). The properties of the reverse reaction suggest that reaction reversal occurs after product release from the resolution reaction, rather than in the context of a post-resolution synaptic complex of the hp telomeres. Temperature plays a key role in regulating the extent of reaction reversal observed. Low-temperature incubation (8 °C) suppresses the forward reaction more than the reverse reaction, allowing for an accumulation of the reversal product (an rTel). This effect is likely mediated by the presence in the hp telomeres of the pre-distorted structure near the cleavage site (i.e. the hairpin turnaround). Formation of this structure by the hairpin-binding module confers cold sensitivity to the cleavage step of the resolution (forward) reaction.

At first sight, telomere fusion seems like it should be heavily disfavoured in vitro due to the entropic cost of bringing two hp telomeres together for their fusion. However, the presence in the product of the hp imparts a higher energy state to the hp telomeres. This finding, combined with the fact that ResT binds to the hp products with equal affinity to that for the rTel, as opposed to higher affinity, somewhat offsets the large bias the reaction would normally display against reversal (Kobryn and Chaconas 2005). The ability of ResT to cleave and fuse its hp telomere products raises the issue of how these activities are restrained in vivo. Frequent hp telomere cleavage or telomere fusions followed by mutation or deletion of the resulting junctions would lead to high levels of inviability or genome instability (i.e. duplications, translocations etc.). The linear replicons of B. burgdorferi appear to be undergoing just these kinds of events, especially near the telomeres (Casjens et al. 2000; Huang et al. 2004b). It has been proposed that action of the telomere resolvase on its product may explain these events (for a discussion of this issue, see Chaconas 2005; Kobryn and Chaconas 2005).

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