Tandemly repeated DNA is a target for the partial replacement of thymine by beta-D-glucosyl-hydroxymethyluracil in Trypanosoma brucei.
In the DNA of African trypanosomes a small fraction of thymine is replaced by the modified base beta-D-glucosyl-hydroxymethyluracil (J). The function of this large base is unknown. The presence of J in the silent variant surface glycoprotein gene expression sites and the lack of J in the transcribed expression site indicates that DNA modification might play a role in control of gene repression. However, the abundance of J in the long telomeric repeat tracts and in subtelomeric arrays of simple repeats suggests that J may also have specific functions in repetitive DNA. We have now analyzed chromosome-internal repetitive sequences in the genome of Trypanosoma brucei and found J in the minichromosomal 177-bp repeats, in the long arrays of 5S RNA gene repeats, and in the spliced-leader RNA gene repeats. No J was found in the rDNA locus or in dispersed repetitive transposon-like elements. Remarkably, the rDNA of T. brucei is not organized in long arrays of tandem repeats, as in many other eukaryotes. T. brucei contains only approximately 15-20 rDNA repeat units that are divided over six to seven chromosomes. Our results show that J is present in many tandemly repeated sequences, either at a telomere or chromosome internal. The presence of J might help to stabilize the long arrays of repeats in the genome.
van Leeuwen
F
F
Division of Molecular Biology and Centre of Biomedical Genetics, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
Kieft
R
R
Cross
M
M
Borst
P
P
D000818:Animals; D004251:DNA Transposable Elements / Q000235:genetics; D016054:DNA, Protozoan / Q000032:analysis / Q000737:chemistry* / Q000235:genetics; D004275:DNA, Ribosomal / Q000032:analysis / Q000235:genetics; D017125:Genes, Protozoan; D020459:Genes, rRNA; D005960:Glucosides / Q000032:analysis*; D012341:RNA, Ribosomal, 5S / Q000235:genetics; D020038:RNA, Spliced Leader / Q000235:genetics; D020080:Tandem Repeat Sequences / Q000235:genetics*; D016615:Telomere / Q000235:genetics; D013941:Thymine / Q000032:analysis*; D014346:Trypanosoma brucei brucei / Q000737:chemistry / Q000235:genetics*; D014498:Uracil / Q000031:analogs & derivatives* / Q000032:analysis
D004251:DNA Transposable Elements; D016054:DNA, Protozoan; D004275:DNA, Ribosomal; D005960:Glucosides; D012341:RNA, Ribosomal, 5S; D020038:RNA, Spliced Leader; C084880:5-((glucopyranosyloxy)methyl)uracil; D014498:Uracil; D013941:Thymine