biophysics. For the articles (i) “Direct observation of RuvAB-catalyzed branch migration of single Holliday junctions,” by Roee Amit, Opher Gileadi, and Joel Stavans, which appeared in issue 32, August 10, 2004, of Proc. Natl. Acad. Sci. USA (101, 11605–11610; first published August 3, 2004; 10.1073/pnas.0404332101), and (ii) “Single-molecule study of RuvAB-mediated Holliday-junction migration,” by A. Dawid, V. Croquette, M. Grigoriev, and F. Heslot, which appeared in issue 32, August 10, 2004, of Proc. Natl. Acad. Sci. USA (101, 11611–11616; first published August 3, 2004; 10.1073/pnas.0404369101), the authors of both papers note the following. “In both articles, the branch migration activity of the molecular motor RuvAB on single Holliday junction structures was measured by the same micromanipulation technique. Surprisingly, the results found by each group concerning the nominal enzyme kinetics seem to differ by a factor of 2 (98 bp/s and 43 bp/s).
“In fact this discrepancy is only apparent: it results from the use of different conventions for the definition of the speed of strand exchange. Indeed, without clearly stating it in their article, Amit et al. measured the variations of the total DNA length of two opposite arms of the cruciform DNA structure (i.e., the bead–surface distance, corrected by the relative extension of the DNA to deduce the contour length (J.S., personal communication), whereas Dawid et al. refer only to one of the two opposite arms (i.e., half the total DNA length between the bead and the surface). Consequently, the results are essentially the same: 49 ± 1.5 bp/s and 43 ± 4 bp/s. This accordance is not clear in the abstract (and even upon detailed reading of both articles) and could generate serious confusion for the reader and possible misinterpretations of the results.”