Trypanosome antigenic variation, involving differential expression of variant surface glycoprotein (VSG) genes, has a strong association with telomeres and with DNA recombination. All expressed VSGs are telomeric, and differential activation involves recombination into the telomeric environment or silencing/activation of subtelomeric promoters. A number of pathogen contingency gene systems associated with immune evasion involve telomeric loci, which has prompted speculation that chromosome ends provide conditions conducive for the operation of rapid gene switching mechanisms. Ku is a protein associated with eukaryotic telomeres that is directly involved in DNA recombination and in gene silencing. We have tested the hypothesis that Ku in trypanosomes is centrally involved in differential VSG expression. We show, via the generation of null mutants, that trypanosome Ku is closely involved in telomere length maintenance, more so for a transcriptionally active than an inactive telomere, but exhibits no detectable influence on DNA double strand break repair. The absence of Ku and the consequent great shortening of telomeres had no detectable influence either on the rate of VSG switching or on the silencing of the telomeric promoters of theVSG subset that is expressed in the tsetse fly. Trypanosome antigenic variation, involving differential expression of variant surface glycoprotein (VSG) genes, has a strong association with telomeres and with DNA recombination. All expressed VSGs are telomeric, and differential activation involves recombination into the telomeric environment or silencing/activation of subtelomeric promoters. A number of pathogen contingency gene systems associated with immune evasion involve telomeric loci, which has prompted speculation that chromosome ends provide conditions conducive for the operation of rapid gene switching mechanisms. Ku is a protein associated with eukaryotic telomeres that is directly involved in DNA recombination and in gene silencing. We have tested the hypothesis that Ku in trypanosomes is centrally involved in differential VSG expression. We show, via the generation of null mutants, that trypanosome Ku is closely involved in telomere length maintenance, more so for a transcriptionally active than an inactive telomere, but exhibits no detectable influence on DNA double strand break repair. The absence of Ku and the consequent great shortening of telomeres had no detectable influence either on the rate of VSG switching or on the silencing of the telomeric promoters of theVSG subset that is expressed in the tsetse fly. non-homologous end joining telomere position effect variant surface glycoprotein bloodstream expression sites open reading frame reverse transcription methyl methanesulfonate primary homology regions wild type The heterodimeric protein Ku, which consists of the subunits Ku70 and Ku80 (or Ku86), associates tightly in a sequence-independent fashion with free ends of double strand DNA and has been associated with a range of nuclear functions in different eukaryotes (reviewed in Refs. 1Featherstone C. Jackson S.P. Mutat. Res. 1999; 434: 3-15Crossref PubMed Scopus (241) Google Scholar, 2Shore D. Curr. Opin. Genet. Dev. 2001; 11: 189-198Crossref PubMed Scopus (83) Google Scholar, 3Dubrana K. Perrod S. Gasser S.M. Curr. Opin. 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Shannon K.B. Weaver D.T. Mol. Cell. Biol. 1996; 16: 4189-4198Crossref PubMed Scopus (259) Google Scholar). A main function of Ku is to bind to DNA and recruit proteins, such as the catalytic subunit of mammalian DNA-dependent protein kinase, that catalyze steps in the repair process, although a weak helicase activity has been ascribed by some to Ku itself (1Featherstone C. Jackson S.P. Mutat. Res. 1999; 434: 3-15Crossref PubMed Scopus (241) Google Scholar). Ku also plays a major role in telomere length maintenance in yeasts such that the deletion of Ku genes causes shortening of the telomere tract (9Boulton S.J. Jackson S.P. EMBO J. 1996; 15: 5093-5103Crossref PubMed Scopus (421) Google Scholar, 10Porter S.E. Greenwell P.W. Ritchie K.B. Petes T.D. Nucleic Acids Res. 1996; 24: 582-585Crossref PubMed Scopus (203) Google Scholar, 11Manolis K.G. Nimmo E.R. Hartsuiker E. Carr A.M. Jeggo P.A. Allshire R.C. EMBO J. 2001; 20: 210-221Crossref PubMed Scopus (134) Google Scholar). 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Nature. 2001; PubMed Scopus Google are in the trypanosome Ku70 in the of the that Although in other there are the and the trypanosome has is that trypanosome Ku80 has a in the an the in the the function of Ku, for in other and the of of either KU to null revealed that and are genes not an during gene genes in bloodstream of T. in In the which is with the and genes in its transcriptionally active telomere, the the the and had the with the the and A number of of of the of in A for that by This the and from which the and The The of wild type trypanosomes and in with of the and as as of the null and Ku is not for bloodstream trypanosome and its absence causes no detectable in that there is a in the of some of the length maintenance on it is to the length of telomeres of the subtelomeric of DNA with a with a but not of a VSG and for that a the telomere tract can be We have gene is in the transcriptionally active telomere in the bloodstream trypanosomes The gene has one in a silent telomere and a that, a is on by the and as as a number of of and for the and and the deletion and We at the effect the absence of Ku had on the telomere of the transcriptionally active expression for and from a more for In from and the wild type and a of also by with and mutants, generally than for or wild type an of telomere maintenance or an in telomere the of telomere in KU mutants, and to be that this to the of Ku, wild type Ku into its a as a to the null and for this of the active expression again to that in telomere than the null for an during for and more in the absence of at which they for telomere tract the for the transcriptionally active a in trypanosomes and in the This of tract We at the effect Ku had on a transcriptionally inactive telomere This with and In this of the in The gene has one in a silent telomere and a that, a is on by for the telomere of on the DNA and as for are in and in The telomere of the gene has not been so is in The to which causes and double strand breaks J. Mutat. Res. PubMed Scopus Google Scholar), and to which causes double strand breaks The as for the in the presence of revealed a effect on the of wild type trypanosomes the range The wild type trypanosomes of in the absence of at at and at This with of the where in the absence of at at and at in the absence of at at and at an of for and of and the effect no in in the of the of in the from a in which trypanosomes in of a and the of This is more but also more to trypanosome The no to or to which tested at trypanosomes some of the for KU genes, there is a role in the differential expression of VSG In the bloodstream of T. switching at a rate and is by a number of including transcriptional switching telomeres and gene into The have of activation to be R. P. Mol. Cell. Biol. 1997; 17: PubMed Scopus Google Scholar), a of for in This to and All the rate of which is of wild type there no for a in rate in the absence of of the switching from a of the that there no in the of VSG switching not VSG genes are expressed in the of the trypanosome in the expression of the subset of telomeric genes known genes, by transcriptional In are not but are in tested the hypothesis that the silencing of in the is by a telomere position We have to T. through to no not so have been to in this T. which to the of the the and We in T. in the and and and to for from regions of The of the transcription sites primary The including the primary and The of prevented its use as a so of the trypanosome Although this A and and for of no in wild or for of the of promoters for the We have and by gene the trypanosome homologues of Ku70 and of telomere revealed the telomere shortening that in other but the role in DNA repair by NHEJ not hypothesis that Ku is in or transcriptional not at that the TPE for S. cerevisiae is not for of antigenic to the that homology Ku proteins from different prevents of homology on the Ku J. 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