Transcriptional control: SWItched-on mobility

Dynamic changes in chromatin structure underlie much of transcriptional control in the eukaryotic nucleus. In the hope of understanding how these are effected, attention has focused on the molecular machines that reorganize chromatin. The best-defined of these machines contain members of the SWI2/SNF2 superfamily of ATPases. These enzymes use the energy of ATP hydrolysis to destabilize chromatin structure sufficiently to promote access by the transcriptional machinery. Exactly how chromatin is destabilized has been a matter of controversy. Now, two carefully-controlled biophysical experiments [1,2], using biochemically purified components, have provided important insights into the workings of one member of the SWI2/SNF2 superfamily, known as ISWI. Surprisingly, it turns out that ISWI uses the energy of ATP hydrolysis to move histone octamers along a DNA molecule, while retaining an intact nucleosomal infrastructure [1,2]. A third study extends these observations to the yeast SWI/SNF complex itself [3]. These new observations suggest solutions to several apparent contradictions in earlier results, but also raise new questions about how the molecular machines containing members of the SWI/SNF superfamily of ATPases work to disrupt chromatin.