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The PWI motif is a highly conserved nucleic-acid-binding domain found in several pre-mRNA processing components. Shown here is a ribbon representation of the solution structure of the PWI motif of SRm160 (the SR-related nuclear matrix protein of 160 kD). The motif is composed of an -helical bundle, which cooperates with an adjacent basic domain (not shown) to provide stable binding to nucleic acids. In addition to its function in splicing, Srm160 (blue oval) also stimulates cleavage by the 3'-end processing machinery (green oval). The RNA-binding activity of the SRm160 PWI domain is important for its 3'-end cleavage-stimulatory function. (For details, see Szymczyna et al., p. 461.)