The Leptomonas seymouri spliced leader RNA promoter requires a novel transcription factor

Characterization of a multisubunit transcription factor complex essential for spliced-leader RNA gene transcription in Trypanosoma brucei

In the unicellular human parasites Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp., the spliced-leader (SL) RNA is a key molecule in gene expression donating its 5'-terminal region in SL addition trans splicing of nuclear pre-mRNA. While there is no evidence that this process exists in mammals, it is obligatory in mRNA maturation of trypanosomatid parasites. Hence, throughout their life cycle, these organisms crucially depend on high levels of SL RNA synthesis. As putative SL RNA gene transcription factors, a partially characterized small nuclear RNA-activating protein complex (SNAP(c)) and the TATA-binding protein related factor 4 (TRF4) have been identified thus far. Here, by tagging TRF4 with a novel epitope combination termed PTP, we tandem affinity purified from crude T. brucei extracts a stable and transcriptionally active complex of six proteins. Besides TRF4 these were identified as extremely divergent subunits of SNAP(c) and of transcription factor IIA (TFIIA). The latter finding was unexpected since genome databases of trypanosomatid parasites appeared to lack general class II transcription factors. As we demonstrate, the TRF4/SNAP(c)/TFIIA complex binds specifically to the SL RNA gene promoter upstream sequence element and is absolutely essential for SL RNA gene transcription in vitro.

Failure to detect binding of Trypanosoma brucei SNAPc to U2 and U6 snRNA gene sequences by in vitro transcription competition and pull-down assays

The small nuclear RNA (snRNA)-activating protein complex (SNAP(c)) is a multi-subunit transcription factor characterized in humans and Drosophila melanogaster. It binds to an upstream sequence element (USE) of snRNA gene promoters and activates both RNA polymerase (pol) II and III-mediated transcription of snRNA genes. The first identified and partially characterized transcription factor in a trypanosomatid organism appears to be a SNAP(c) homologue. It was identified in Leptomonas seymouri and shown to specifically interact with the USE of the RNA pol II-transcribed spliced leader (SL) RNA gene. Recently, chromatin immunoprecipitation and a gel shift assay suggested that L. seymouri SNAP(c) also interacts with RNA pol III-transcribed U2 and U6 snRNA genes. Previously, we have characterized and epitope-tagged the Trypanosoma brucei homologue of the SNAP50 subunit. Here, we show by in vitro transcription competition and promoter pull-down assays that TbSNAP50 binds to the SL RNA gene promoter and parasite-specifically to the ribosomal RNA gene promoter. Conversely, we did not detect binding of the factor to U2 and U6 snRNA gene sequences. Since U snRNA gene promoters are structurally conserved among trypanosomatids, our findings contrast those obtained in L. seymouri and suggest that trypanosomatid SNAP(c) is not involved in RNA pol III-mediated transcription of U snRNA genes.

A novel type of single-stranded nucleic acid binding protein recognizing a highly frequent motif in the intergenic regions of Trypanosoma cruzi

Regulation of gene expression in trypanosomatids is not yet well understood. Genes are organized in long polycistronic transcriptional units separated by intergenic regions that may contain the signaling information for nucleic acid processing. Poly-dinucleotides are frequent in these regions and have been proposed to be involved in regulation of gene expression. Previously, we have reported that [dT-dG] are highly frequent, asymmetrically strand distributed, and constitute targets for specific protein binding [Biochem. Biophys. Res. Commun. 287 (2001) 98]. Here, we present the purification and characterization of a new type of single stranded nucleic acid binding protein (Tc38) that recognizes specifically the motif poly[dT-dG] in this parasite. The protein has a deduced molecular weight of 38kDa and its salient characteristics include an isoelectric point of 9.34, a high frequency of Ser, Leu, and di-amino acids. Neither compositional nor architectural conserved domains could be detected in database searches. Recombinant Tc38 was expressed as a GST fusion protein, purified, and used to analyze target specificity by electrophoretic mobility shift assays. The unusual characteristics of the protein together with the peculiar features of the specific nucleic acid target suggest the existence of a novel event that may be involved in the mechanisms of gene expression in trypanosomatids.

The 5' end structure of transcripts derived from the rRNA gene and the RNA polymerase I transcribed protein coding genes in Trypanosoma brucei

Due to trans-splicing and polycistronic transcription, the 5' end structure of precursor RNAs of protein coding genes in Trypanosoma brucei has not yet been characterized. In eukaryotes, in general, the 5' ends of transcripts generated by RNA polymerase (pol) I and pol II are different. Pol I derived precursor RNAs contain an unmodified tri- or diphosphate group at their 5' ends. In contrast, pol II primary transcripts, the 5' triphosphate (initially also part of the pre-mRNA) is rapidly modified by the addition of methylated guanosine triphosphate, immediately after transcription initiation. We determined the 5' end structure of precursor RNAs of the rRNA gene and the RNA pol I transcribed protein coding gene by the differential display of RNA ligase mediated amplification of cDNA ends (DDRLACE) method. Comparing the ability of the 5' end of RNA transcripts to ligate with an RNA primer following different pre-treatments, the structure of the 5' end of RNA transcripts was characterized. We found that: (1). the 5' end of putative precursor RNAs from a pol I transcribed protein coding gene and the rRNA gene was uncapped; (2). approximately 20% of the putative rRNA precursor contained a 5' tri- or diphosphate group, representing the primary transcript and approximately 80% of the putative rRNA precursor were dephosphorylated and contained a 5' hydroxyl group; (3). the majority of putative neomycin resistance gene precursor RNAs, driven by the procyclin gene promoter (a pol I promoter), contained a 5' hydroxyl group. The procyclin-neo primary transcript, as being those containing a 5' tri- or diphosphate, was below a detectable level in the steady state RNA; and (4). we did not detect pol I transcribed precursor RNAs that contained a 5' monophosphate group. The observation that the putative pre-RNAs derived from the procyclin gene promoter, similar to those of rRNA do not have a 5' capped structure, is consistent with the notion that transcription of pol I transcribed protein coding genes is crucially dependent on trans-splicing for the cap addition.