A cis-acting A-U sequence element induces kinetoplastid U-insertions

The RNA binding activity of the first identified trypanosome protein with Z-DNA-binding domains

RNA-binding proteins play a particularly important role in regulating gene expression in trypanosomes. A map of the network of protein complexes in Trypanosoma brucei uncovered an essential protein (Tb927.10.7910) that is postulated to be an RNA-binding protein implicated in the regulation of the mitochondrial post-transcriptional gene regulatory network by its association with proteins that participate in a multi-protein RNA editing complex. However, the mechanism by which this protein interacts with its multiple target transcripts remained unknown. Using sensitive database searches and experimental data, we identify Z-DNA-binding domains in T. brucei in the N- and C-terminal regions of Tb927.10.7910. RNA-binding studies of the wild-type protein, now referred to as RBP7910 (RNA binding protein 7910), and site-directed mutagenesis of residues important for the Z-DNA binding domains show that it preferentially interacts with RNA molecules containing poly(U) and poly(AU)-rich sequences. The interaction of RBP7910 with these regions may be involved in regulation of RNA editing of mitochondrial transcripts.

Identification of specific inhibitors for a trypanosomatid RNA editing reaction

Several mitochondrial mRNAs of the trypanosomatid protozoa are edited through the post-transcriptional insertion and deletion of uridylates. The reaction has provided insights into basic cellular biology and is also important as a potential therapeutic target for the diseases caused by trypanosomatid pathogens. Despite this importance, the field has been hindered by the lack of specific inhibitors that could be used as probes of the reaction mechanism or developed into novel therapeutics. In this study, an electrochemiluminescent aptamer-switch was utilized in a high-throughput screen for inhibitors of a trypanosomatid RNA editing reaction. The screen identified GW5074, mitoxantrone, NF 023, protoporphyrin IX, and D-sphingosine as inhibitors of insertion editing, with IC(50) values ranging from 1 to 3 μM. GW5074 and protoporphyrin IX are demonstrated to inhibit at or before the endonuclease cleavage that initiates editing and will be valuable biochemical probes for the early events of the in vitro reaction. Since protoporphyrin IX and sphingosine are both naturally present within the trypanosomatids, their effectiveness as in vitro inhibitors is also suggestive of the potential for in vivo modulatory roles.

An electrochemiluminescent aptamer switch for a high-throughput assay of an RNA editing reaction

An RNA editing reaction that is both essential and specific to the trypanosomatid parasites is an attractive target for new drug development. Although high-throughput screening of chemical libraries is a powerful strategy often used to identify new drugs, the available in vitro editing assays do not have the necessary sensitivity and format for this approach to be feasible. A ruthenium labeled reporter RNA is described here that overcomes these limitations as it can both detect edited product in the low femtomole range and is ideal for high-throughput format. The reporter RNA consists of an RNA editing substrate linked to a streptavidin-binding aptamer that is initially held within an inactive conformation. An in vitro selection strategy optimized the linkage so that the streptavidin-binding aptamer is only activated by an editing-induced conformational change. An electrochemiluminescent signal results from the ruthenium label when the reporter is bound to the bottom of a streptavidin-coated microtiter plate where it can be stimulated by a carbon electrode. Chemical probing, mutagenesis, and binding affinity measurements were used to characterize the reporter. The highly sensitive assay could be adapted to a broad range of RNA processing reactions.

Cis-acting elements stimulating kinetoplastid guide RNA-directed editing

The coding sequence of several mitochondrial mRNAs of the kinetoplastid protozoa is created through the insertion and deletion of specific uridylates. The editing reactions are required to be highly specific in order to ensure that functional open reading frames are created in edited mRNAs and that potentially deleterious modification of normally nonedited sequence does not occur. Selection-amplification and mutagenesis were previously used to identify the optimal sequence requirements for in vitro editing. There is, however, a minority of natural editing sites with suboptimal sequence. Several cis-acting elements, obtained from an in vitro selection, are described here that are able to compensate for a suboptimal editing site. An A + U sequence element within the 5'-untranslated region of cytochrome b mRNA from Leishmania tarentolae is also demonstrated to function as a cis-acting guide RNA and is postulated to compensate for a suboptimal editing site in vivo. Two proteins within an enriched editing extract are UV-cross-linked to two different in vitro selected editing substrates more efficiently than poorly edited RNAs. The results suggest that these proteins contribute to the specificity of the editing reaction.

Sequence and structural requirements for optimal guide RNA-directed insertional editing within Leishmania tarentolae

The coding sequence of several mitochondrial mRNAs of the trypanosomatid family of protozoa is created by the guide RNA-directed insertion and deletion of uridylates (Us). Selection-amplification was used to explore the sequence and structure of the guide RNA and mRNA required for efficient insertional editing within a mitochondrial extract prepared from Leishmania tarentolae. This study identifies several novel features of the editing reaction in addition to several that are consistent with the previous mutagenesis and phylogenetic analysis of the reaction in Trypanosoma brucei, a distantly related trypanosomatid. Specifically, there is a strong bias against cytidines 5' of the editing sites and guanosines immediately 3' of guiding nucleotides. U insertions are directed both 5' and 3' of a genomically encoded U, which was previously assumed not to occur. Base pairing immediately flanking an editing site can significantly stimulate the editing reaction and affect the reaction fidelity but is not essential. Likewise, single-stranded RNA in the region upstream of the editing site, not necessarily immediately adjacent, can facilitate editing but is also not essential. The editing of an RNA containing many of the optimal features is linear with increasing quantities of extract permitting specific activity measurements to be made that are not possible with previously described T. brucei and L. tarentolae assays. The reaction catalyzed by the L. tarentolae extract can be highly accurate, which does not support a proposed model for editing that was based largely on the inaccuracy of an earlier in vitro reaction.

Uridine insertion/deletion RNA editing in trypanosome mitochondria: a complex business

The basic mechanism of uridine insertion/deletion RNA editing in mitochondria of kinetoplastid protists has been established for some time but the molecular details remained largely unknown. Recently, there has been significant progress in defining the molecular components of the editing reaction. A number of factors have been isolated from trypanosome mitochondria, some of which have been definitely implicated in the uridine insertion/deletion RNA editing reaction and others of which have been circumstantially implicated. Several protein complexes have been isolated which exhibit some editing activities, and the macromolecular organization of these complexes is being analyzed. In addition, there have been several important technical advances in the in vitro analysis of editing. In this review we critically examine the various factors and complexes proposed to be involved in RNA editing.

Direct visualisation of RNA editing within a Leishmania tarentolae mitochondrial extract

The coding sequence within several mitochondrial mRNAs of the trypanosomatid protozoa is created through editing by the precise insertion and deletion of U nucleotides. The biochemical characterisation of the editing reaction in the Leishmania genus of the trypanosomatids has been hindered by the lack of a direct in vitro assay. We describe here the first direct assay for the detection of guide RNA-directed editing mediated by a mitochondrial extract prepared from two independent isolates of Leishmania tarentolae. The assay enabled the editing activity within a L. tarentolae mitochondrial extract to be significantly enriched and will facilitate the characterisation of the editing reaction. The results suggest that the difficulty in establishing an assay for the L. tarentolae reaction was not simply a result of the catalytic machinery being limiting but rather reflected the presence of constraints on both the guide RNA and mRNA sequences.

A mRNA determinant of gRNA-directed kinetoplastid editing

Several mitochondrial mRNAs of the kinetoplastid protozoa do not encode a functional open reading frame until they have been edited through the addition or deletion of U nucleotides at specific sites. Genetic information specifying the location and extent of editing is present on guide RNAs (gRNAs). The sequence adjacent to most mRNA editing sites has a high purine content which previously has been proposed to facilitate the editing reaction through base-pairing to a poly(U) tail at the 3' end of the gRNA. We demonstrate here that gRNA binding alone is insufficient to create an editing site and that the mRNA sequence near an editing site is an additional determinant affecting the efficiency of the reaction.

Functions and mechanisms of RNA editing

RNA editing can be broadly defined as any site-specific alteration in an RNA sequence that could have been copied from the template, excluding changes due to processes such as RNA splicing and polyadenylation. Changes in gene expression attributed to editing have been described in organisms from unicellular protozoa to man, and can affect the mRNAs, tRNAs, and rRNAs present in all cellular compartments. These sequence revisions, which include both the insertion and deletion of nucleotides, and the conversion of one base to another, involve a wide range of largely unrelated mechanisms. Recent advances in the development of in vitro editing and transgenic systems for these varied modifications have provided a better understanding of similarities and differences between the biochemical strategies, regulatory sequences, and cellular factors responsible for such RNA processing events.

Activation of guide RNA-directed editing of a cytochrome b mRNA

The coding sequence of several mitochondrial mRNAs of the kinetoplastid protozoa is created only after the addition or deletion of specific uridines. Although in vitro systems have been valuable in characterizing the editing mechanism, only a limited number of mRNAs are accurately edited in vitro. We demonstrate here that in vitro editing of cytochrome b mRNA is inhibited by an A-U sequence present on both the 5'-untranslated sequence and on a cytochrome b guide RNA. Mutation of the sequence on the guide RNA stimulates directed editing and results in the loss of binding to at least one component within the editing extract. Mutation of the sequence on the mRNA increases the accuracy of the editing. Evidence is provided that suggests the A-U sequence interacts with the editing machinery both in vitro and in vivo.

Uridine insertion/deletion RNA editing in trypanosome mitochondria--a review

The uridine insertion/deletion RNA editing in trypanosome mitochondria is a unique posttranscriptional RNA maturation process that involves the addition or removal of uridine residues at precise sites usually within the coding regions of mitochondrial transcripts. This process creates initiation and termination codons, corrects frameshifts and even builds entire open-reading frames from nonsense sequences. The development of several in-vitro editing assays has provided much insight into the molecular mechanism of RNA editing, which appears to involve cleavage, U addition, exonuclease trimming and ligation, essentially as proposed in the original 'enzyme cascade' model (Blum, B., Bakalara, N., Simpson, L., 1990. A model for RNA editing in kinetoplastid mitochondria: 'Guide' RNA molecules transcribed from maxicircle DNA provide the edited information. Cell 60, 189-198). However, little is known about the biochemical properties of the proteins involved and the significance and role of this process. This article is a review of recent findings on uridine-insertion/deletion editing in trypanosome mitochondria, with an emphasis on the proteins isolated and characterized that may have a role in this process.