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Mol. Cell. Biol., Mar 1997, 1264-1273, Vol 17, No. 3
BK Singleton, A Priestley, H Steingrimsdottir, D Gell, T Blunt, SP Jackson, AR Lehmann and PA Jeggo
The gene product defective in radiosensitive CHO mutants belonging to
ionizing radiation complementation group 5, which includes the extensively
studied xrs mutants, has recently been identified as Ku80, a subunit of the
Ku protein and a component of DNA-dependent protein kinase (DNA-PK).
Several group 5 mutants, including xrs-5 and -6, lack double-stranded DNA
end-binding and DNA-PK activities. In this study, we examined additional
xrs mutants at the molecular and biochemical levels. All mutants examined
have low or undetectable levels of Ku70 and Ku80 protein, end-binding, and
DNA-PK activities. Only one mutant, xrs-6, has Ku80 transcript levels
detectable by Northern hybridization, but Ku80 mRNA was detectable by
reverse transcription-PCR in most other mutants. Two mutants, xrs-4 and -6,
have altered Ku80 transcripts resulting from mutational changes in the
genomic Ku80 sequence affecting RNA splicing, indicating that the defects
in these mutants lie in the Ku80 gene rather than a gene controlling its
expression. Neither of these two mutants has detectable wild-type Ku80
transcript. Since the mutation in both xrs-4 and xrs-6 cells results in
severely truncated Ku80 protein, both are likely candidates to be null
mutants. Azacytidine-induced revertants of xrs-4 and -6 carried both
wild-type and mutant transcripts. The results with these revertants
strongly support our model proposed earlier, that CHO-K1 cells carry a copy
of the Ku80 gene (XRCC5) silenced by hypermethylation. Site-directed
mutagenesis studies indicate that previously proposed ATP-binding and
phosphorylation sites are not required for Ku80 activity, whereas N-
terminal deletions of more than the first seven amino acids result in
severe loss of activities.
Copyright © 1997, American Society for Microbiology
Molecular and biochemical characterization of xrs mutants defective in Ku80
MRC Cell Mutation Unit, University of Sussex, Brighton, United Kingdom.
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