Detection and characterization of DNA polymerase activity in Toxoplasma gondii

Genetic rescue of a Toxoplasma gondii conditional cell cycle mutant

Growth rate is a major pathogenesis factor in the parasite Toxoplasma gondii; however, how cell division is controlled in this protozoan is poorly understood. Herein, we show that centrosomal duplication is an indicator of S phase entry while centrosome migration marks mitotic entry. Using the pattern of centrosomal replication, we confirmed that mutant ts11C9 undergoes a bimodal cell cycle arrest that is characterized by two subpopulations containing either single or duplicated centrosomes which correlate with the bipartite genome distribution observed at the non-permissive temperature. Genetic rescue of ts11C9 was performed using a parental RH strain cDNA library, and the cDNA responsible for conferring temperature resistance (growth at 40 degrees C) was recovered by recombination cloning. A single T. gondii gene encoding the protein homologue of XPMC2 was responsible for genetic rescue of the temperature-sensitive defect in ts11C9 parasites. This protein is a known suppressor of mitotic defects, and in tachyzoites, TgXPMC2-YFP localized to the parasite nucleus and nucleolus which is consistent with the expected subcellular localization of critical mitotic factors. Altogether, these results demonstrate that ts11C9 is a conditional mitotic mutant containing a single defect which influences two distinct control points in the T. gondii tachyzoite cell cycle.

DNA replication and daughter cell budding are not tightly linked in the protozoan parasite Toxoplasma gondii

In the protozoan parasite Toxoplasma gondii, cell division occurs by an unusual internal budding process whereby two daughter cells develop within and eventually subsume the mother cell. We have examined this process using inhibitors targeted at specific events in the cell cycle. By adding inhibitors to newly established parasites we were able to examine the effects of the inhibitors on parasites treated at the start of intracellular development and many hours prior to the onset of daughter cell budding. As with other eukaryotes, inhibitors of nuclear DNA synthesis blocked parasite DNA synthesis and prevented cell division. Examination of parasites treated with the nuclear DNA synthesis inhibitor aphidicolin showed that the formation of daughter apical complexes and the initiation of budding occurred as normal and only the inability of the nucleus to become incorporated into the daughter cells prevented successful cell division. Moreover, these inhibitory effects of aphidicolin were not reversible. The initiation of nuclear DNA synthesis and cell division in newly invaded Toxoplasma required both gene transcription and protein synthesis, although inhibitors of mitochondrial DNA synthesis, transcription and protein synthesis did not block parasite division. Thus, unlike most eukaryotes, Toxoplasma tachyzoites have separated nuclear DNA replication and mitosis from the events associated with cell division (daughter cell budding). This implies that Toxoplasma tachyzoites may have dispensed with specific cell cycle checkpoints present in other eukaryotes with, in particular, a DNA-replication checkpoint control either missing, or downregulated in this stage of the parasite life cycle.

Partial purification and characterization of mitochondrial DNA polymerase from Plasmodium falciparum

Mitochondria of chloroquine-resistant Plasmodium falciparum (K1 strain) were isolated from mature trophozoites by differential centrifugation. The mitochondrial marker enzyme cytochrome c reductase was employed to monitor the steps of mitochondria isolation. Partial purification of DNA polymerase from P. falciparum mitochondria was performed using fast protein liquid chromatography (FPLC). DNA polymerase of P. falciparum mitochondria was characterized as a gamma-like DNA polymerase based on its sensitivity to the inhibitors aphidicolin, N-ethylmaleimide and 9-beta-D-arabinofuranosyladenine-5'-triphosphate. In contrast, the enzyme was found to be strongly resistant to 2',3'-dideoxythymidine-5'-triphosphate (IC(50)>400 microM) and differed in this aspect from the human homologue, possibly indicating structural differences between human and P. falciparum DNA polymerase gamma. In addition, the DNA polymerase of parasite mitochondria was shown to be resistant (IC(50)>1 mM) to the nucleotide analogue (S)-1-[3-hydroxy-2-phosphonylmethoxypropyl]adenine diphosphate (HPMPApp).

Isolation and partial characterization of three DNA polymerases from Trypanosoma cruzi

Three distinct DNA polymerase fractions (A, B and C), were isolated from Trypanosoma cruzi epimastigote forms. Fraction A is a low molecular mass enzyme corresponding to beta-like DNA polymerase of T. cruzi. Fraction B co-purified along several purification steps with fraction A, but in the last step it was clearly separated by a phosphocellulose chromatography. Fraction C was separated from fractions A and B by binding to DEAE-cellulose column, since the other two fractions were eluted in the flowthrough. This enzyme has an apparent native molecular mass of 100 kDa and showed a high preference for poly(dC)-oligo(dG) among different template-primers tested as substrate. Western-blot and biochemical analysis strongly suggest that the three DNA polymerase fractions correspond to different molecular entities. These results are in agreement with the idea that fraction C is a new DNA polymerase of T. cruzi, not described before.

Characterization of the gene encoding the catalytic subunit of the DNA polymerase alpha from Toxoplasma gondii

The gene encoding the catalytic subunit of the Toxoplasma gondii DNA polymerase alpha enzyme has been isolated. The coding region is 6487 bp in length, containing three introns, and specifies a protein of 1690 aa. The seven conserved regions which characterize the polalpha polypeptide, as well as four of the five polalpha-specific aa domains, were found in the T. gondii gene. The absence of one of these domains, as well as the presence of a unique cysteine cluster between domains IV and B in the T. gondii polalpha, may result in a slight difference in the secondary or even tertiary structure compared with the human homologue and thus may be suitable for designing anti-Toxoplasma drugs. A number of amino acid differences within the seven conserved regions between the human and T. gondii polalpha, as well as variations in the spacings of these regions, were also observed.

Detection and characterization of DNA polymerase activity in Entamoeba histolytica

DNA polymerase activity was detected and characterized in nuclear extracts from trophozoites of Entamoeba histolytica. The activity was high at pH 2 to pH 6, but at pH 8 and 10 the activity was very low. The presence of K+ was inhibitory for the activity and a higher concentration of K+ markedly inhibited the activity. Magnesium ions (Mg2+) were absolutely required for activity and its optimal concentration was 6 to 8 mM. The activity was markedly inhibited by aphidicolin which is an inhibitor of mammalian DNA polymerases alpha, delta, and epsilon and also by N-ethylmaleimide which is an inhibitor of DNA polymerases, alpha, gamma, delta and epsilon. However, inhibition of the activity by 2', 3'-dideoxythymidine-5'-triphosphate which is an inhibitor of DNA polymerases beta and gamma was relatively weak. Thus sensitivity of the E. histolytica enzyme to these inhibitors was similar to that of mammalian DNA polymerases (alpha, delta and epsilon) of the alpha family. Monoclonal antibodies against human DNA polymerase alpha did not bind to DNA polymerase of E. histolytica.