A ribosomal RNA gene promoter at the telomere of a mini-chromosome inTrypanosoma brucei

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Abstract
The parasitic protozoan Trypanosoma brucei has some hundred mini-chromosomes of 50–150 kb, which mainly consist of telomeric repeats, sub-telomeric repeats and internal 177-bp repeats. Their primary function seems to be to expand the repertoire of non-transcribed sub-telomeric variant surface glycoprotein (VSG) genes. Here we report that two of the smaller mini-chromosomes (55 and 60 kb) contain sequences homologous to the ribosomal RNA gene promoter region. We have targeted by homologous recombination the neomycin phosphotransferase (neo 1 ) gene behind the promoter on the 55 kb chromosome and show that this promoter mediates the efficient synthesis of properly trans-spliced and polyadenylated neo mRNA. The resulting high resistance to G418 (a neo analogue) is stable in the absence of drug showing that mitotic segregation of this mini-chromosome is precise. Downstream of the transcription start the wild-type version of the ribosomal promoter is flanked by telomeric repeats. The absence of the sub-telomeric repeats found in other T.brucel chromosome ends suggests that the rDNA-telomere junction has been formed by de novo addition of telomeric repeats to a broken chromosome end (healing). Our results provide a plausible explanation for the α-amanltin-resistant transcription of telomeric repeats in T.brucei reported by Rudenko and Van der Ploeg 1 and they show that trypanosomes can efficiently use RNA polymerase I for the expression of sub-telomeric genes, supporting the notion that the α-amanitin-resistant transcription of sub-telomeric VSG genes may also be catalyzed by this enzyme.