The role of recombination in hairpin telomere maintenance.

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Borrelia burgdorferi, is a well-established natural science model system for the study of DNA hairpin (hp) telomeres. Hairpin telomeres are an unusual solution to the problem of copying linear DNAs. Hairpin telomeres are closed hairpins at linear chromosome or plasmid termini. They eliminate the free end normally present in linear DNAs and, thereby, eliminate the copying problem that would result in the progressive loss of information at the DNA ends every time the cells divide. DNA copying in the linear chromosome is bidirectional and starts in the middle of the DNA and proceeds towards the two hp telomeres. Replication, around the hp telomeres, would produce circular dimer daughter DNAs linked together via replicated telomere (rTel) junctions. In order for the daughter DNAs to be divided between the daughter cells the circular dimer is processed by a specialized enzyme, ResT. ResT performs DNA cleavage and rejoining reactions at the rTel junctions that separates linear daughter DNAs regenerating new hp telomeres at the linear DNA ends. My lab studies the properties of ResT. In this proposal we will examine/challenge the assumption of the above model that rTel junctions are always produced by normal DNA copying functions, in a continuation of DNA synthesis around the hp telomeres. In recent progress in the lab we have discovered that ResT has unexpected biochemical properties that allow it to separate DNA strands and re-anneal them. We have also discovered that ResT is associated with a protein that promotes exchange of genetic information between copies of the same DNA. We have named this protein RarA (for ResT-associated recombinase A). These activities are very similar to those of proteins involved, in many organisms, with the repair of DNA damage and the repair of stalled DNA copying complexes at sites of DNA damage. We hypothesize that these similarities to repair enzymes indicate that the hp telomeres look to the cell like DNA damage and that functions provided by ResT/RarA are needed to complete the copying of Borrelia's linear DNAs. We will test our hypothesis by looking at the properties of the proteins in the test tube and in Borrelia grown in the lab. We will compare the properties of ResT and RarA to known repair enzymes in the test tube to determine the extent of the similarities with the known repair functions. Then we will interfere with the function of these proteins in the cell to see if this interrupts the cell's ability to copy their DNA. If these proteins have DNA repair-like functions important for cell survival then they would constitute the first identified examples of these repair functions in Borrelia. It was previously thought that Borrelia lacked these functions. Study of similar fundamental issues involved in copying the linear DNA of yeast and mammalian cells has contributed to huge advances in our understanding of fundamental biological issues like cellular transformation and aging.