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 surf ace glycoprotein gene expression sites and the lack of J in the transcribed expression site indicates that DNA modification might play a role in contr ol of gene repression. However, the abundance of J in the long telomeric re peat tracts and in subtelomeric arrays of simple repeats suggests that J ma y also have specific functions in repetitive DNA. We have now analyzed chro mosome-internal repetitive sequences in the genome of Trypanosoma brucei an d found J in the minichromosomal 177-bp repeats, in the long arrays of 5S R NA 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, Rem arkably. the rDNA of T. brucei is not organized in long arrays of tandem re peats, as in many other eukaryotes. T. brucei contains only similar to 15-2 0 rDNA repeat units that are divided over six to seven chromosomes. Our res ults show that J is present in many tandemly repeated sequences, either at a telomere or chromosome internal. The presence of J might help to stabiliz e the long arrays of repeats in the genome. (C) 2000 Elsevier Science B.V. All rights reserved.