Characterization of satellite DNA in Trypanosoma brucei and Trypanosoma cruzi

Abstract

We have determined the properties of the simple-sequence satellite DNAs from two protozoa, Trypanosoma brucei and Trypanosoma cruzi. The T. brucei satellite DNA contains 29 mol% guanine plus cytosine and is made up of long tandem arrays of a 177 base-pair repeat. Sequence heterogeneity in these repeats is limited and restricted to certain positions as shown by sequence analysis, restriction enzyme digestion and two-dimensional analysis of nucleotides bordering the AluI and HhaI recognition sites in the repeat. The repeat contains two copies of a 19 base-pair sequence differing by a single base-pair substitution and several additional copies of part of this sequence. Sequence variants of the repeat are clustered in the DNA. Satellite DNA is not detectably linked to other DNA and no transcripts of this DNA are found in T. brucei. The T. cruzi satellite DNA repeat is 196 base-pairs long and contains 53 mol% guanine plus cytosine. Direct repetitions longer than eight base-pairs were not observed in the nucleotide sequence of this repeat. The nucleotide sequences of the satellites of T. brucei and T. cruzi are not related. In cell fractionation experiments, the T. brucei and T. cruzi satellite DNAs were recovered from the nuclear fraction. Micrococcal nuclease digestion of nuclear fractions yielded 193 and 197 base-pair nucleosomal oligomers in T. brucei and T. cruzi, respectively; these oligomers contained satellite DNA but not the extranuclear kinetoplast DNA. The 193 base-pair nucleosomal repeat of T. brucei is significantly different from the 177 base-pair satellite repeat. Satellite and nucleosomal repeats are, therefore, not in phase in T. brucei. These satellite DNAs are the first to be observed in protozoa, and we conclude that their properties are similar to those of satellites from animals or plants.