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GENOME RECONSTITUTION AFTER EXTENSIVE DNA DAMAGE IN
Deinococcus radiodurans
John R. Battista1 and Michael M. Cox2
1
Department of Biological Sciences, Louisiana State University and A & M College,
Baton Rouge, LA 70803; Department of Biochemistry, 2University of Wisconsin –
Madison, Madison, Wisconsin 53706-1544;
The bacterium Deinococcus radiodurans can withstand extraordinary levels of
ionizing radiation, reflecting an equally extraordinary capacity for DNA repair. This
bacterium is 200 times more radiation resistant than E. coli. D. radiodurans is
multiploid, maintaining 4-10 copies of each of its four chromosomes at all times. After
treatment with 5000Gy of ionizing radiation, Deinococcus radiodurans suffers about
180 DNA double strand breaks per cell. Over the next six hours, new chromosomes are
reconstructed from overlapping fragments. This reconstitution of the bacterial genome
is both highly accurate and efficient.
There are at least four distinct mechanisms contributing to genome
reconstitution. First, the genome is organized in the form of a tightly structured toroid
which may contribute to genome reconstitution in a manner not yet defined (Englander
et al. (2004) J. Bact. 186, 5973). In addition, there are at least three repair processes
involved. Five novel Deinococcus genes involved in genome restoration (ddrA, ddrB,
ddrC, ddrD, and pprA) define three epistasis groups. The first two involve repair
processes that are RecA-independent. One, defined by the ddrA gene and protein, is a
DNA end-protection system that prevents nuclease-mediated degradation of
chromosomal DNA fragments. The second, defined by the ddrB gene, promotes a
degree of homology-dependent genome reconstitution and may be a single-strand
annealing pathway. The pprA gene product participate in a third, RecA-dependent
process during recovery from radiation damage.
The RecA, SSB, DdrA, and PprA proteins of Deinococcus radiodurans have
been purified and characterized in vitro. Each offers a novel perspective on the
mechanisms for chromosomal repair in this bacterium. These in vivo and in vitro
characterizations demonstrate that novel mechanisms contribute to the ionizing
radiation resistance in D. radiodurans.