Variations in frequencies of drug resistance in Plasmodium falciparum

Continual exposure of malarial parasite populations to different drugs may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials. To test this hypothesis, different Plasmodium falciparum clones having varying numbers of preexisting resistance mechanisms were treated with two new antimalarial agents: 5-fluoroorotate and atovaquone. All parasite populations were equally susceptible in small numbers. However, when large populations of these clones were challenged with either of the two compounds, significant variations in frequencies of resistance became apparent. On one extreme, clone D6 from West Africa, which was sensitive to all traditional antimalarial agents, failed to develop resistance under simple nonmutagenic conditions in vitro. In sharp contrast, the Indochina clone W2, which was known to be resistant to all traditional antimalarial drugs, independently acquired resistance to both new compounds as much as a 1,000 times more frequently than D6. Additional clones that were resistant to some (but not all) traditional antimalarial agents acquired resistance to atovaquone at high frequency, but not to 5-fluoroorotate. These findings were unexpected and surprising based on current views of the evolution of drug resistance in P. falciparum populations. Such new phenotypes, named accelerated resistance to multiple drugs (ARMD), raise important questions about the genetic and biochemical mechanisms related to the initiation of drug resistance in malarial parasites. Some potential mechanisms underlying ARMD phenotypes have public health implications that are ominous.

Kinetics of formation of hypoxanthine containing base pairs by HIV-RT: RNA template effects on the base substitution frequencies

Hypoxanthine (H), the deamination product of adenine, has been implicated in the high frequency of A to G transitions observed in retroviral and other RNA genomes. Although H·C base pairs are thermodynamically more stable than other H·N pairs, polymerase selection may be determined in part by kinetic factors. Therefore, the hypoxanthine induced substitution pattern resulting from replication by viral polymerases may be more complex than that predicted from thermodynamics. We have examined the steady-state kinetics of formation of base pairs opposite template H in RNA by HIV-RT, and for the incorporation of dITP during first- and second-strand synthesis. Hypoxanthine in an RNA template enhances the k2app for pairing with standard dNTPs by factors of 10–1000 relative to adenine at the same sequence position. The order of base pairing preferences for H in RNA was observed to be H·C >> H·T > H·A > H·G. Steady-state kinetics of insertion for all possible mispairs formed with dITP were examined on RNA and DNA templates of identical sequence. Insertion of dITP opposite all bases occurs 2–20 times more frequently on RNA templates. This bias for higher insertion frequencies on RNA relative to DNA templates is also observed for formation of mispairs at template A. This kinetic advantage afforded by RNA templates for mismatches and pairing involving H suggests a higher induction of mutations at adenines during first-strand synthesis by HIV-RT.

High frequency of sex and equal frequencies of mating types in natural populations of the ciliate Tetrahymena thermophila.

In ciliate protists, sex involves the temporary joining of two cells of compatible mating type, followed by meiosis and exchange of gametic nuclei between conjugants. Reproduction is by asexual binary fission following conjugation. For the many ciliates with fixed multiple mating types, frequency-dependent sex-ratio theory predicts equal frequencies of mating types, if sex is common in nature. Here, we report that in natural populations of Tetrahymena thermophila sexually immature cells, indicative of recent conjugation, are found from spring through fall. In addition, the seven mating types occur in approximately equal frequencies, and these frequencies appear to be maintained by interaction between complex, multiple mat alleles and environmental conditions during conjugation. Such genotype-environment interaction determining mating type frequency is rare among ciliates.