Materials

With the exception of avian myeloblastosis virus reverse transcriptase（Seikagaku America，Inc.，St.Petersburg，F L），all restriction endonucleases or DNA-modifying enzymes were the products of either Bethesda Research Laboratories or New England Biolabs（Beverly，M A）.GeneScreen and 32PO4（carrier free）were from New England Nuclear，and［α-32P］dCTP（8000 ci/mmol）was from Amersham Corp.Dextran sulfate was obtained from Pharmacia Fine Chemicals，and nitrocellulose was supplied by Schleicher and Schuell.With the exception of phenol，which was distilled before use，all other chemicals were of reagent grade and were used without further purification.Synthetic deoxyoligonucleotides were synthesized at the Biotechnology Center on the campus of the University of Illinois at Urbana-Champaign on an Applied Biosystems model 380A DNA synthesizer.(https://www.daowen.com)