Fig. 1. Differential macroarray screen. (A) Digital image of macroarray colony filters hybridized with a radiolabeled RNA probe synthesized from LiCl-treated embryos (Before subtraction) and with a radiolabeled RNA probe synthesized after subtractive hybridization (After subtraction). Before the subtractive hybridization, two different 500 nucleotide bacteriophage sequences (₁ and ₂) were added to the selectate at a concentration equivalent to five copies/average cell and were also spotted onto the library filters. A magnification of the spots corresponding to the and ubiquitin clones are shown. The clones became detectable only after subtraction while the ubiquitin sequence, common to both selectate and driver, was not enriched after subtraction. (B) Table showing the level of enrichment obtained by the subtractive hybridization, measured by QPCR as enrichment of the sequences relative to the ubiquitin sequence. QPCR amplification of the and ubiquitin sequences was carried out on an aliquot of the subtractive hybridization reaction before HAP chromatography (Before HAP) and on the single-strand fraction after HAP chromatography (After HAP). The differences in cycle threshold (Ct) between the sequences and ubiquitin were calculated (Ct Ubiq); Ct values were means of triplicates. The enrichment values are the cycle amplification efficiency (1.9) raised to the power of the difference [(before HAP CtUbiq)-(after HAP CtUbiq)].