The first-strand full-length cDNA/mRNA hybrid was captured on magnetic porous glass particles coated with streptavidin (CPG, NJ, USA). Before binding of the nucleic acids, 500 μl of beads (1% suspension; 1 mg of beads can bind 800 pmol of a biotinylated 25-mer oligonucleotide) was blocked by adding 2.5 μl of 40 μg μl–1 DNA-free tRNA and incubating on ice for 30 min with occasional gentle vortexing. Just before nucleic acid capture, the beads were separated using a magnetic stand, and the supernatant was removed by pipetting. All subsequent capture, washing and release procedures were performed with the help of a magnetic stand. After the blocking step, the beads were washed three times with 500 μl of 2 m NaCl and 50 mm EDTA (pH 8.0) and finally resuspended in 400 μl of 2 m NaCl and 50 mm EDTA (pH 8.0). Finally, cDNA (approximately 0.5 μg) was captured at room temperature for 30 min with continuous gentle mixing in the presence of 100 μg of tRNA as a carrier to prevent bead sedimentation. After removal of unbound cDNA, the beads were washed twice with 2 m NaCl and 50 mm EDTA, followed by 1 washing with 0.4% SDS and 50 μg ml–1 yeast tRNA; 1 washing with 10 mm Tris–HCl (pH 7.5), 0.2 mm EDTA, 10 mm NaCl, 20% glycerol, one washing with nuclease-free water containing 50 μg ml–1 yeast tRNA; and, finally, 1 washing with 1 X RNase H buffer (20 mm Tris–HCl (pH 7.5), 10 mm MgCl2, 20 mm KCl, 0.1 mm EDTA, 0.1 mm (DTT). To release full-length cDNA from the beads, the sample was incubated with three units of RNase H in 100 μl of RNase H buffer at 37°C for 30 min with continuous mixing, followed by the addition of 0.1% SDS and 10 mm EDTA and additional incubation at 65°C for 10 min. To remove the cDNA fraction which was not removed from the beads because of incomplete RNase H treatment, additional alkaline hydrolysis was performed in the presence of 100 μl of Tris-formate buffer (pH 9.0) (obtained by combining 100 mm Tris base with 16.6 mm formic acid, 0.016 mm EDTA, and 0.1% SDS, all at their final concentrations) at 65°C for 10 min. The cycle of alkaline elution was repeated four times. The alkaline-treated fraction was removed and processed with the RNase H-released fraction. One microgram of glycogen was added to the single-stranded cDNA sample. The cDNA was ethanol-precipitated under standard conditions (Sambrook et al. 1989), dissolved in 50 μl of water, and subjected to G100 Sephadex column chromatography. The radioactive fractions were collected in siliconized Eppendorf tubes, 1 μg of glycogen was added, and the cDNA was ethanol-precipitated and dissolved in 31 μl of water.