Trypanosoma brucei: cis-acting sequences involved in the developmental regulation of PARP expression

The zinc finger proteins ZC3H20 and ZC3H21 stabilise mRNAs encoding membrane proteins and mitochondrial proteins in insect-form Trypanosoma brucei

ZC3H20 and ZC3H21 are related trypanosome proteins with two C(x)₈ C(x)₅ C(x)₃ H zinc finger motifs. ZC3H20 is present at a low level in replicating mammalian-infective bloodstream forms, but becomes more abundant when they undergo growth arrest at high density; ZC3H21 appears only in the procyclic form of the parasite, which infects Tsetse flies. Each protein binds to several hundred mRNAs, with overlapping but not identical specificities. Both increase expression of bound mRNAs, probably through recruitment of the MKT1-PBP1 complex. At least 28 of the bound mRNAs decrease after depletion of ZC3H20, or of ZC3H20 and ZC3H21 together; their products include procyclic-specific proteins of the plasma membrane and energy metabolism. Simultaneous depletion of ZC3H20 and ZC3H21 causes procyclic forms to shrink and stop growing; in addition to decreases in target mRNAs, there are other changes suggestive of loss of developmental regulation. The bloodstream-form-specific protein RBP10 controls ZC3H20 and ZC3H21 expression. Interestingly, some ZC3H20/21 target mRNAs also bind to and are repressed by RBP10, allowing for dynamic regulation as RBP10 decreases and ZC3H20 and ZC3H21 increase during differentiation.

Untranslated regions of mRNA and their role in regulation of gene expression in protozoan parasites

Protozoan parasites are one of the oldest living entities in this world that throughout their existence have shown excellent resilience to the odds of survival and have adapted beautifully to ever changing rigors of the environment. In view of the dynamic environment encountered by them throughout their life cycle, and in establishing pathogenesis, it is unsurprising that modulation of gene expression plays a fundamental role in their survival. In higher eukaryotes, untranslated regions (UTRs) of transcripts are one of the crucial regulators of gene expression (influencing mRNA stability and translation efficiency). Parasitic protozoan genome studies have led to the characterization (in silico, in vitro and in vivo) of a large number of their genes. Comparison of higher eukaryotic UTRs with parasitic protozoan UTRs reveals the existence of several similar and dissimilar facets of the UTRs. This review focuses on the elements of UTRs of medically important protozoan parasites and their regulatory role in gene expression. Such information may be useful to researchers in designing gene targeting strategies linked with perturbation of host-parasite relationships leading to control of specific parasites.

Trypanosoma brucei: a putative RNA polymerase II promoter

RNA polymerase II (pol II) promoters are rare in the African trypanosome Trypanosoma brucei because gene regulation in the parasite is complex and polycistronic. Here, we describe a putative pol II promoter and its structure-function relationship. The promoter has features of an archetypal eukaryotic pol II promoter including putative canonical CCAAT and TATA boxes, and an initiator element. However, the spatial arrangement of these elements is only similar to yeast pol II promoters. Deletion mapping and transcription assays enabled delineation of a minimal promoter that could drive orientation-independent reporter gene expression suggesting that it may be a bidirectional promoter. In vitro transcription in a heterologous nuclear extract revealed that the promoter can be recognized by the basal eukaryotic transcription complex. This suggests that the transcription machinery in the parasite may be very similar to those of other eukaryotes.

Presence of multiple mRNA cycling sequence element-binding proteins in Crithidia fasciculata

A consensus sequence present in the 5'- or 3'-untranslated regions of several Crithidia fasciculata messenger RNAs encoding proteins involved in DNA metabolism has been shown to be necessary for the periodic accumulation of these mRNAs during the cell cycle. A protein complex termed cycling sequence-binding protein (CSBP) has two subunits, CSBPA and CSBPB, and binds the consensus sequence with high specificity. The binding activity of CSBP was shown to vary during the cell cycle in parallel with the levels of putative target mRNAs. Although disruption of the CSBPA gene resulted in loss of both CSBPA and CSBPB, the putative target message levels still continued to vary during the cell cycle. The presence of an additional and distinct binding activity was revealed in these CSBPA null mutant cells. This activity, termed CSBP II, was also expressed in wild-type Crithidia cells. CSBP II has higher binding specificity for the cycling sequence element than the earlier described CSBP complex. Three polypeptides associated with purified CSBP II show specific binding to the cycling sequence. These proteins may represent a family of sequence-specific RNA-binding proteins involved in post-transcriptional regulation.

Stage-specific expression and targeting of cyst wall protein-green fluorescent protein chimeras in Giardia

In preparation for being shed into the environment as infectious cysts, trophozoites of Giardia spp. synthesize and deposit large amounts of extracellular matrix into a resistant extracellular cyst wall. Functional aspects of this developmentally regulated process were investigated by expressing a series of chimeric cyst wall protein 1 (CWP1)-green fluorescent protein (GFP) reporter proteins. It was demonstrated that a short 110 bp 5' flanking region of the CWP1 gene harbors all necessary cis-DNA elements for strictly encystation-specific expression of a reporter during in vitro encystation, whereas sequences in the 3' flanking region are involved in modulation of steady-state levels of its mRNA during encystation. Encysting Giardia expressing CWP1-GFP chimeras showed formation and maturation of labeled dense granule-like vesicles and subsequent incorporation of GFP-tagged protein into the cyst wall, dependent on which domains of CWP1 were included. The N-terminal domain of CWP1 was required for targeting GFP to regulated compartments of the secretory apparatus, whereas a central domain containing leucine-rich repeats mediated association of the chimera with the extracellular cyst wall. We show that analysis of protein transport using GFP-tagged molecules is feasible in an anaerobic organism and provides a useful tool for investigating the organization of primitive eukaryotic vesicular transport.

The procyclin repertoire of Trypanosoma brucei. Identification and structural characterization of the Glu-Pro-rich polypeptides

The surface of the insect stages of the protozoan parasite Trypanosoma brucei is covered by abundant glycosyl phosphatidylinositol (GPI)-anchored glycoproteins known as procyclins. One type of procyclin, the EP isoform, is predicted to have 22-30 Glu-Pro (EP) repeats in its C-terminal domain and is encoded by multiple genes. Because of the similarity of the EP isoform sequences and the heterogeneity of their GPI anchors, it has been impossible to separate and characterize these polypeptides by standard protein fractionation techniques. To facilitate their structural and functional characterization, we used a combination of matrix-assisted laser desorption ionization and electrospray mass spectrometry to analyze the entire procyclin repertoire expressed on the trypanosome cell. This analysis, which required removal of the GPI anchors by aqueous hydrofluoric acid treatment and cleavage at aspartate-proline bonds by mild acid hydrolysis, provided precise information about the glycosylation state and the number of Glu-Pro repeats in these proteins. Using this methodology we detected in a T. brucei clone the glycosylated products of the EP3 gene and two different products of the EP1 gene (EP1-1 and EP1-2). Furthermore, only low amounts of the nonglycosylated products of the GPEET and EP2 genes were detected. Because all procyclin genes are transcribed polycistronically, the latter finding indicates that the expression of the GPEET and EP2 genes is post-transcriptionaly regulated. This is the first time that the whole procyclin repertoire from procyclic trypanosomes has been characterized at the protein level.