The RNA-dependent RNA polymerase, QDE-1, is a component of the RNA silencing pathway in Neurospora crassa. The enzymatically active carboxy-terminal fragment QDE-1 DeltaN has been expressed in Saccharomyces ceresiviae in the presence and absence of selenomethionine (SeMet). The level of SeMet incorporation was estimated by mass spectrometry to be similar to98%. Both native and SeMet proteins were crystallized in space group P2(1) with unit cell parameters a = 101.2, b = 122.5. c = 114.4 Angstrom, beta = 108.9degrees, and 2 molecules per asymmetric unit. The native and SeMet crystals diffract to 2.3 and 3.2 Angstrom, respectively, the latter are suitable for MAD structure determination. (C) 2004 Elsevier Inc. All rights reserved.