In vitro transcription DNA templates were linearized by BamHI digestion and purified by successive phenol/chloroform extraction and column filtration (Qiagen). SP6 polymerase directed transcription was carried out in a 50-ml reaction containing 1-3 mg linearized DNA, 1 mM NTPs, 40 mM Tris-HCl (pH 7.9), 10 mM NaCl, 10 mM DTT, 6 mM MgCl 2 , 2 mM spermidine, 6 units of Rnase inhibitor (RNAguard, Pharmacia) and 15 units of SP6 polymerase (Promega). 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 desalted and sterilized water were directly used for in vitro translations. Alternatively, transcripts were purified by phenol/ chloroform extraction and isopropanol precipitation as described earlier (27) .