Structural differences between the chromatin of procyclic Trypanosoma brucei brucei and of higher eukaryotes as probed by immobilized trypsin

Histone divergence in trypanosomes results in unique alterations to nucleosome structure

Eukaryotes have a multitude of diverse mechanisms for organising and using their genomes, but the histones that make up chromatin are highly conserved. Unusually, histones from kinetoplastids are highly divergent. The structural and functional consequences of this variation are unknown. Here, we have biochemically and structurally characterised nucleosome core particles (NCPs) from the kinetoplastid parasite Trypanosoma brucei. A structure of the T. brucei NCP reveals that global histone architecture is conserved, but specific sequence alterations lead to distinct DNA and protein interaction interfaces. The T. brucei NCP is unstable and has weakened overall DNA binding. However, dramatic changes at the H2A-H2B interface introduce local reinforcement of DNA contacts. The T. brucei acidic patch has altered topology and is refractory to known binders, indicating that the nature of chromatin interactions in T. brucei may be unique. Overall, our results provide a detailed molecular basis for understanding evolutionary divergence in chromatin structure.

Histone genes in trypanosomatids

Histone genes in Trypanosomatids are of considerable interest because these flagellates do not condense their chromatin during mitosis. In contrast to higher eukaryotes, histone genes in Trypanosomatids are found on separate chromosomes, and their transcripts are polyadenylated. Sequence similarity of Trypanosomatid core histones with those of higher eukaryotes is found predominantly in the globular region; the N-terminal is highly divergent. Finally, in general, Trypanosomatid histones H1 are of low molecular weight, bearing closest homology to the C-terminal region of the higher eukaryote histones H1. These features constitute interesting targets for a rational approach to the study of these protozoa, as discussed here by Norbel Galanti and colleagues.

Isolation and characterization of subnuclear compartments from Trypanosoma brucei. Identification of a major repetitive nuclear lamina component

Protozoan parasites of the order Kinetoplastida are responsible for a significant proportion of global morbidity and economic hardship. These organisms also represent extremely distal points within the Eukarya, and one such organism, Trypanosoma brucei, has emerged as a major system for the study of evolutionary cell biology. Significant technical challenges have hampered the full exploitation of this organism, but advances in genomics and proteomics provide a novel approach to acquiring rapid functional data. However, the vast evolutionary distance between trypanosomes and the higher eukaryotes presents significant problems with functional assignment based on sequence similarity, and frequently homologues cannot be identified with sufficient confidence to be informative. Direct identification of proteins in isolated organelles has the potential of providing robust functional insight and is a powerful approach for initial assignment. We have selected the nucleus of T. brucei as a first target for protozoan organellar proteomics. Our purification methodology was able to reliably provide both nuclear and subnuclear fractions. Analysis by gel electrophoresis, electron microscopy, and immunoblotting against trypanosome subcellular markers indicated that the preparations are of high yield and purity, maintain native morphology, and are well resolved from other organelles. Minor developmental differences were observed in the nuclear proteome for the bloodstream and procyclic stages, whereas significant morphological alterations were visible. We demonstrate by direct sequencing that the NUP-1 nuclear envelope antigen is a coiled coil protein, containing approximately 20 near-perfect copies of a 144-amino acid sequence. Immunoelectron microscopy localized NUP-1 to the inner face of the nuclear envelope, suggesting that it is a major filamentous component of the trypanosome nuclear lamina.

Chromatin remodelling during the life cycle of trypanosomatids

The mechanisms which control the expression of developmentally regulated genes in trypanosomatids remain unclear. The genes are grouped together into transcription units that are co-transcribed to yield polycistronic RNAs. Trans-splicing and polyadenylation give rise to mature, monocistronic mRNAs. It is difficult to imagine that expression of these genes is controlled at the level of transcription initiation because this would suggest that the genes are transcribed at the same rate. This is not the case, because at any given developmental stage in trypanosomes or Leishmania, genes transcribed from the same transcription unit are expressed at different levels within the cell. Consequently, these parasites must rely on post-transcriptional or post-translational mechanisms to generate the appropriate levels of gene product within the cell. There are no well-established examples of RNA polymerase II promoters in trypanosomes or Leishmania. However, the promoters for genes encoding the variant surface glycoprotein (VSG) and the procyclic acidic repetitive protein (PARP) have been identified and resemble ribosomal RNA polymerase I promoters. In higher eukaryotes where the mechanisms regulating transcription are clearer, there is increasing evidence that epigenetic factors, such as histones and modified bases, influence gene expression. Chemical modification of these factors can restructure chromatin and lead to gene activation or silencing. In trypanosomatids, an epigenetic mechanism for the control of developmentally expressed genes is a possibility. In this review, chromatin remodelling during the life and cell cycle of trypanosomes and Leishmania is explored, and the influence of epigenetic factors such as histones and modified bases on this process is discussed.

Characterization of the histones of Trypanosoma brucei brucei bloodstream forms

Five groups of histones were shown in Trypanosoma brucei brucei, displaying qualitative and quantitative differences between two stages of the parasite's life-cycle. The influence of the histones of T. b. brucei bloodstream forms on the compaction pattern of the chromatin was investigated and their extractability in diluted acids and their amino acid composition were analysed. While nonhistone proteins barely influenced the formation of higher-order chromatin structures, the histone H1-like proteins were essential for the regular spacing of the nucleosomes and the salt-dependent condensation of the nucleosome filament. Differences were seen in the amino-acid composition of histones of bloodstream forms as compared to procyclic culture forms and higher eukaryotes which may explain the disparities seen in the condensation of the chromatin between the two stages of the life cycle as well as the lack of a salt-dependent condensation into a 30 nm fiber. They point to an alternative method of organizing and processing the genetic information in the nucleus of the trypanosome as compared to higher eukaryotes, the possible hosts of the parasite.

The chromatin of trypanosomes

The nuclear chromatin of trypanosomes is organised in the form of nucleosome filaments. When soluble chromatin is prepared under suitable conditions, a regular array of nucleosomes can be shown by electron microscopy. Chromatin of blood stream as well as procyclic culture forms of Trypanosoma brucei brucei and of T. cruzi shows limited compaction at salt concentrations increasing from 1 to 100 mM. No 30 nm fibres, typical for higher eukaryotes, are formed. Digestion of the nuclear chromatin with micrococcal nuclease and analysis of the histone proteins with various techniques reveal that the basic organisation of the trypanosome chromatin is similar but not identical as compared to that of higher eukaryotes. Distinct differences are present with respect to biochemical properties of the histones as well as to their interaction with the DNA. The primary structure of the histones also differs significantly from that found in other lower and higher eukaryotes. The function of the recently described H1-like proteins in trypanosomes is currently being investigated. The differences that have already been found in the structure and compaction of the trypanosome chromatin compared to that of higher eukaryotes lead us to expect differences of gene expression which, in turn, might offer targets for the control of trypanosomiasis.

Structural analysis of Trypanosoma brucei brucei chromatin by limited proteolysis. An electrical-birefringence study

The sensitive electric-birefringence method was used to reveal structural differences between the soluble chromatin of procyclic Trypanosoma brucei brucei and the chromatin of the higher eukaryotes. The orientation of the nucleosomal chains and the presence of extended DNA were analysed from the sign and amplitude of the steady-state birefringence, and the conformational properties (overall dimensions and flexibility) were studied in relation to the orientational relaxation times. In contrast to the higher eukaryotes, the birefringence of T. brucei brucei is negative and of low amplitude, corresponding to that of H1-depleted rat liver nucleosomes. Furthermore, the relaxation times are very small, about 10 microseconds. If salt is added, the birefringence as well as the relaxation time decreases dramatically, indicating that condensation affects T. brucei brucei chromatin although it behaves like nucleosome filaments, with less stable DNA-protein interaction than for the higher eukaryotes. However, this condensation does not induce the formation of regular higher-order structure. This complies with the hypothesis that typical histone H1 is absent from T. brucei brucei chromatin and that a protein or protein domain fulfils the role of histone H1. The accessibility and structural role of histone-like proteins in T. brucei brucei chromatin were also investigated using limited proteolysis with enzymes covalently bound to nylon spheres. The analysis of protein products obtained after digestion with immobilized trypsin and subtilisin shows that proteins a and d, which are classified as H3 and H4 histones, respectively, are the first to be attacked. The changes in chromatin conformation indicate that chromatin undergoes a structural transition, leading to decondensation, as indicated by increases in negative birefringence and relaxation time, and to a change in its orientation mechanism, indicated by the appearance of a permanent moment. This result is very interesting since, in rat liver, H4 was very resistant and was the last histone to be attacked, suggesting internal location and its involvement in nucleosome stabilization rather than higher-order condensation. Therefore, in T. brucei brucei chromatin, the characteristic properties of proteins a and d (their composition and interaction with DNA), as well as their external location on the nucleosome surface, suggest that if these proteins play a role similar to that played by H3 and H4 in higher eukaryotes, probably through their N-terminal regions and interaction either with DNA or protein domains, the mechanisms involved in chromatin compaction are quite different.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of antagonists of polyamine metabolism on African trypanosomes

This review outlines the metabolism of polyamines in African trypanosomes and summarizes evidence to indicate that trypanosome polyamine metabolism differs in several important aspects from that of the mammalian host. These differences relate to the halflife, turnover, substrate specificity and regulation of enzymes within the mainstream of polyamine synthesis and the related pathway of transmethylation. The common denominator for the uniqueness of parasite polyamine metabolism concerns S-adenosylmethionine (AdoMet) whose synthesis is unregulated and, upon accumulating in the cell, appears to result in abnormally high transmethylation activity. Similarly, the catabolism of the AdoMet product of polyamine synthesis, methylthioadenosine, is governed by a phosphorylase having broad substrate specificity, and which, if presented with substrate analogs, can generate cytotoxic metabolites.

Trypanosoma brucei brucei: differences in the nuclear chromatin of bloodstream forms and procyclic culture forms

Nucleosome filaments of two stages of the life-cycle of Trypanosoma brucei brucei, namely bloodstream forms and procyclic culture forms, were investigated by electron microscopy. Chromatin of bloodstream forms showed a salt-dependent condensation. The level of condensation was higher than that shown by chromatin from procyclic culture forms, but 30 nm fibres as formed in rat liver chromatin preparations were not found. Analysis of histones provided new evidence for the existence of H1-like proteins, which comigrated in the region of the core histones in SDS-PAGE and in front of the core histones in Triton acid urea gels. Differences were found between the H1-like proteins of the two trypanosome stages as well as between the core histones in their amount, number of bands and banding pattern. It can be concluded that T. b. brucei contains a full set of histones, including H1-like proteins, and that the poor condensation of its chromatin is not due to the absence of H1, but most probably due to histone-DNA interaction being weak. It is obvious that structural and functional differences of the chromatin exist not only between T. b. brucei and higher eukaryotes, but also between various stages of the life-cycle of the parasite. It is therefore not adequate to investigate the chromatin only of the procyclic culture forms as a model for all stages of the life-cycle of T. b. brucei.

Biochemical and functional characterization of histone H1-like proteins in procyclic Trypanosoma brucei brucei

Four variants and/or posttranslational modifications of histone H1-like proteins of Trypanosoma brucei brucei procyclic culture forms were extracted with 0.25 N HCl from isolated nuclei and analyzed by two-dimensional gel electrophoresis. The amino acid composition of these proteins, their ability to space nucleosomes regularly and to induce salt-dependent condensation of the chromatin indicated their histone H1 nature. On the other hand, the histone H1-like proteins clearly differed from their higher-eukaryote counterparts by their weak interaction with DNA under low-salt conditions. As a consequence, intact nucleosome filaments were prepared according to a new preparation protocol especially adapted to the unstable chromatin of T. b. brucei. Our results indicate that the biochemical properties of the histone H1-like proteins contribute to the structural and functional differences between the chromatin of procyclic T. b. brucei and that of higher eukaryotes.