Tying a molecular knot with optical tweezers.

Filamentous structures are abundant in cells. Relatively rigid filaments, such as microtubules and actin, serve as intracellular scaffolds that support movement and force, and their mechanical properties are crucial to their function in the cell. Some aspects of the behaviour of DNA, meanwhile, depend critically on its flexibility-for example, DNA-binding proteins can induce sharp bends in the helix. The mechanical characterization of such filaments has generally been conducted without controlling the filament shape, by the observation of thermal motions or of the response to external forces or flows. Controlled buckling of a microtubule has been reported, but the analysis of the buckled shape was complicated. Here we report the continuous control of the radius of curvature of a molecular strand by tying a knot in it, using optical tweezers to manipulate the strand's ends. We find that actin filaments break at the knot when the knot diameter falls below 0.4 microm. The pulling force at breakage is around 1 pN, two orders of magnitude smaller than the tensile stress of a straight filament. The flexural rigidity of the filament remained unchanged down to this diameter. We have also knotted a single DNA molecule, opening up the possibility of studying curvature-dependent interactions with associated proteins. We find that the knotted DNA is stronger than actin.     

Malaria. Cooperative silencing elements in var genes

Each Plasmodium falciparum malaria parasite carries about 50 var genes from a diverse family that encode variable adhesion proteins on the infected red blood cells of the host, but individual parasites single out just one var gene for expression and silence all the others. Here we show that this silencing is established during the DNA-synthesis phase (S phase) of the cell cycle and that it depends on the cooperative interaction between two elements in separate control regions of each var gene (the 5'-flanking region and the intron). This finding should help to clarify the mechanisms by which parasites coordinate the silencing and activation of var genes that are responsible for antigenic variation in malaria.     

Malaria research in the post-genomic era

For many pathogens the availability of genome sequence, permitting genome-dependent methods of research, can partially substitute for powerful forward genetic methods (genome-independent) that have advanced model organism research for decades. In 2002 the genome sequence of Plasmodium falciparum, the parasite causing the most severe type of human malaria, was completed, eliminating many of the barriers to performing state-of-the-art molecular biological research on malaria parasites. Although new, licensed therapies may not yet have resulted from genome-dependent experiments, they have produced a wealth of new observations about the basic biology of malaria parasites, and it is likely that these will eventually lead to new therapeutic approaches. This review will focus on the basic research discoveries that have depended, in part, on the availability of the Plasmodium genome sequences.