Displacement synthesis of globin complementary DNA: evidence for sequence amplification

We have examined a cDNA displacement synthesis procedure in which the extent of precursor incorporation and the unusual kinetics of displacement synthesis suggest a unique replicative form of DNA and the occurrence of multiple rounds of displacement synthesis, leading to amplification of mRNA sequences. Globin double-stranded DNA containing a hairpin loop was extended by the addition of a homopolymer to the 3' end. This was followed by displacement synthesis with the Klenow fragment of DNA polymerase I that was primed by an oligonucleotide hybridized to the homopolymer. Thus, the hairpin cDNA was copied to form an open duplex with an inverted repetition of globin sequences. These molecules can then serve as templates for additional synthesis which would be primed from oligomers bound the homopolymer. Globin cDNA sequences appear to be amplified 10-fold or more by this procedure. Globin cDNA obtained by displacement synthesis was similar in size to the original template. However, displaced molecules associate to the extent that they are not readily resolved by electrophoresis or sedimentation under nondenaturing conditions. Restriction endonuclease digests of 32P-labeled displaced strands gave fragment patterns similar to rabbit globin cDNA hairpin molecules. S1 nuclease studies demonstrated that displaced complexes and replication intermediates are partially single stranded, which might account for their aggregation properties.