Cloning and characterization of a highly conserved HMG-like protein (PF16) gene from Plasmodium falciparum

A Plasmodium falciparum homologue of the ATPase subunit of a multi-protein complex involved in chromatin remodelling for transcription

A Plasmodium falciparum homologue of one of the components of a chromatin-remodelling complex which controls binding of transcription factors to nucleosome core particles has been cloned and characterised. The gene encodes 1422 amino acids with an estimated molecular mass of 167 kDa. The protein, SNF2L, shares 60% amino acid identity in its conserved DNA-dependent ATPase domain with yeast transcription factors originally identified by characterising mating type switch mutants. It also contains sequences related to the so-called SWI3, ADA2, N-CoR and TFIIIB B" or SANT DNA binding domains which are characteristic of these transcriptional activation factors. The SNF2L gene has two short introns in the 3' region of the coding sequence of the gene and is transcribed into a single approximately 6.5 kb messenger RNA species which is present throughout the asexual stages of the cell cycle. Southern blotting and pulsed field gel electrophoresis experiments show that SNF2L is a single copy gene. located on P. falciparum chromosome 11.

Molecular cloning and characterization of a cDNA for the highly conserved HMG-like protein (Pf16) gene of Plasmodium falciparum

A cDNA clone (PfHB3-2-4) of 1538 bp corresponding to the highly conserved HMG-like protein (Pf16) was isolated. However, northern analysis suggests that the mRNA is about 2.2 to 2.3 kb. Analysis by RT-PCR indicated that the 0.6 to 0.7 kb sequence missing in the cDNA maps to the 3' end, suggesting that the cDNA is terminated within the 26 adenosine residues that are in the middle of the Pf16 sequence. The most unique feature about this cDNA is the presence of two open reading frames (ORF), one from nucleotides 91 to 927 and the other starting from 1421. The second ORF corresponds to Pf16. Expression of the cDNA clones in Escherichia coli and translation in rabbit reticulocytes of RNA transcribed from the T7 promoter of the cDNA clones revealed that only the 3' end Pf16 is translated from this mRNA. Further experiments with antisense oligonucleotides specific for Pf16 indicated that the Pf16 protein serves an important function in the life cycle of the parasite.

Plasmodium falciparum protein kinase 5 and the malarial nuclear division cycles

In the course of our studies on cell cycle regulation mechanisms of Plasmodium falciparum, we investigated expression pattern, kinase activity, and localization of PfPK5, a putative malarial member of the family of cyclin-dependent protein kinase (cdks). The kinase was immunoprecipitated from parasites of selected stages and from parasites blocked with the cell-cycle inhibitor aphidicolin. An elevated kinase activity of PfPK5 from aphidicolin-blocked cells suggested that the enzyme might be implicated in the regulation of the parasite's S-phase. To further investigate this hypothetical function, parasite cultures were treated with the specific cdk inhibitors flavopiridol and olomoucine, which act on PfPK5 in vitro at similar concentrations as on other cdks. When applied during the nuclear division cycles of the parasite, both drugs markedly inhibited the DNA synthesis, as predicted from our proposition that PfPK5 is necessary to activate or maintain the parasite S-phase. Immunolocalization studies provide further evidence for this potential role of PfPK5.

A signature for the HMG-1 box DNA-binding proteins

A diverse group of DNA-binding regulatory proteins share a common structural domain which is homologous to the sequence of a highly conserved and abundant chromosomal protein, HMG-1. Proteins containing this HMG-1 box regulate various cellular functions involving DNA binding, suggesting that the target DNA sequences share a common structural element. Members of this protein family exhibit a dual DNA-binding specificity: each recognizes a unique sequence as well as a common DNA conformation. The highly conserved HMG-1/-2 proteins may modulate the binding of other HMG-1 box proteins to bent DNA. We examine the structural and functional relationships between the proteins, identify their signature and describe common features of their target DNA elements.

Structure and expression of a post-transcriptionally regulated malaria gene encoding a surface protein from the sexual stages of Plasmodium berghei

The sexual stage-specific protein Pbs21 of the rodent malaria parasite Plasmodium berghei, expressed on the surface of zygotes and ookinetes, has been shown to induce an effective and long-lasting transmission blocking immunity. The gene encoding Pbs21 was cloned by screening a cDNA library prepared from enriched zygotes and ookinetes using the monoclonal antibody 13.1.15, which is capable of blocking subsequent parasite sexual development in the mosquito vector. The Pbs21 gene encoded a protein of 213 amino acids which contained a putative amino-terminal signal sequence and a putative carboxy-terminal hydrophobic membrane anchor. The amino-acid sequence was characterised by a large number of cysteine residues which were organized into 4 epidermal growth factor-like domains. The spacing of the cysteine residues was highly conserved when compared to the 25-kDa ookinete proteins of Plasmodium falciparum (Pfs25), Plasmodium reichenowi (Prs25) and Plasmodium gallinaceum (Pgs25) which were approximately 45%, 45% and 40% homologous to Pbs21 respectively. The gene is located on chromosome 5 and cross-hybridizes to a similarly defined gene unit in the other rodent malaria species Plasmodium chabaudi, Plasmodium vinckei and Plasmodium yoelii. The gene is internally disposed and not in the subtelomeric region of chromosome 5. The gene is transcribed in a stage-specific manner giving rise to an abundant 1.5-kb transcript. This mRNA is synthesised in the precursor cells to female gametes (gametocytes) however the protein is observed only after activation of the gametes, suggesting that translation of the mRNA is controlled by a post-transcriptional process. The Pbs21 gene and the P. berghei parasite system provide an excellent vehicle for the study of stage-specific transcriptional and post-transcriptional control in malaria.