The apical organelles of malaria merozoites: host cell selection, invasion, host immunity and immune evasion

Malaria is caused by protozoan parasites belonging to the phylum Apicomplexa. These obligate intracellular parasites depend on the successful invasion of an appropriate host cell for their survival. This article is a broad overview of the molecular strategies employed by the merozoite, an invasive form of the malaria parasite, to successfully invade a suitable red blood cell.

Thiostrepton binds to malarial plastid rRNA

Binding of the thiazolyl peptide antibiotic thiostrepton to the GTPase domain of 23S rRNA involves a few crucial nucleotides, notably A1067 (E. coli). Small RNA transcripts were prepared corresponding to the GTPase domain of the plastid 23S rRNA and the two forms of cytosolic 28S rRNAs found in the human malaria parasite Plasmodium falciparum, as well as the plastid form of rRNA of the AIDS-related pathogen Toxoplasma gondii. Binding affinities of the wild type and mutated RNA sequences were as predicted; the malarial plastid sequence had by far the highest affinity, whereas that from toxoplasma did not bind thiostrepton.

tRNA genes transcribed from the plastid-like DNA of Plasmodium falciparum

Besides their mitochondrial genome, malarial parasites contain a second organellar DNA. This 35 kb circular molecule has a number of features reminiscent of plastid DNAs. Sequence analysis shows that along with other genes the circle codes for 25 different tRNAs all of which are transcribed. Six of the tRNAs have some unusual features, and one has an intron, the only one found so far on the circle. Comparison of codon and anticodon usage indicates that the 25 tRNAs are sufficient to decode all the protein genes present on the circle. The maintenance of such a parsimonious but complete translation system is further evidence for the functionality of the circle.

Nine duplicated tRNA genes on the plastid-like DNA of the malaria parasite Plasmodium falciparum

A major feature of the plastid-like circular DNA of Plasmodium falciparum is an inverted repeat comprising duplicated genes for rRNA (rrn) and tRNA (trn). We have identified nine putative trn genes in each arm of the repeat on the basis of their potential clover-leaf structures and conserved residues. Northern blots indicate that these trn genes are expressed.

The evolutionary origin of the 35 kb circular DNA of Plasmodium falciparum: new evidence supports a possible rhodophyte ancestry

In common with other Apicomplexan parasites, Plasmodium falciparum carries two extrachromosomal DNAs, one of which, the 6 kb element, is undoubtedly mitochondrial. The second, generally referred to as the 35 kb circle, is of unknown provenance, but the nature and organization of its genetic content makes a mitochondrial association unlikely and the molecule has features reminiscent of plastid genomes. We now report the occurrence on the circle of an open reading frame specifying a predicted 470 amino acid protein that shares more than 50% identity with a gene currently known only on the plastome of red algae. This high degree of conservation confirms the 35 kb circle's plastid ancestry, and we speculate that it may have originated from the rhodoplast of an ancient red algal endosymbiont in the progenitor of the Apicomplexa.

Malaria and other Apicomplexans: the "plant" connection

Recent molecular studies point to the unorthodox conclusion that malaria parasites have a photosynthetic ancestry. Coupled with other phylogenetic evidence, this finding may apply across the phylum Apicomplexa, including other important pathogens such as the coccidia and piroplasms. We propose that an organelle corresponding to a residual plastid, performing unknown functions, is likely to reside in all, or many, of these organisms. The new findings discussed here highlight once more the pressing need to discover more about the basic biology of these economically important parasites. From such knowledge, new targets for chemotherapy may be identified.

Poly(U)-binding protein inhibits Drosophila pre-5 S RNA 3'-exonuclease digestion

A approximately 50-kDa protein binds specifically to the 3' terminus of 135-nucleotide Drosophila pre-5 S RNA. Unlabeled poly(U) competes out protein binding and stimulates the activity of a 3'-exonuclease, which eventually degrades the substrate to 120 nucleotides, the size of mature 5 S RNA. In its RNA binding and UV cross-linking properties, the endogenous poly(U)-binding protein resembles human La, an autoantigen that binds the U > 3 3' ends of vertebrate RNA polymerase III primary transcripts. This protein appears to inhibit a 3' exonuclease and could protect 5 S RNA for faithful processing and transport.