DNA templates were linearized by BamHI digestion and purified by successive phenol/chloroform extraction and column filtration (Qiagen, Benelux). SP6 polymerase directed transcriptions were carried out in 50 ml reactions containing $2 mg linearized DNA, 10 mM NTPs, 40 mM Tris-HCl (pH 7.9), 10 mM NaCl, 10 mM DTT, 6 mM MgCl 2 , 2 mM spermidine, 6 U of RNase inhibitor (RNAsin, Promega, Benelux) and 15 U of SP6 polymerase (Promega, Benelux). After an incubation period of 2 h at 37 C, samples were taken and run on agarose gels to determine the quality and quantity of the transcripts. Appropriate dilutions of the reaction mix in water were directly used for in vitro translations. Alternatively, transcripts were purified by phenol/chloroform extraction and isopropanol precipitation and quantified by UV absorption as described previously (14) .